[docs] Fix 404 link for testing example.

[leerob]

Closes #33406.

This PR is a demonstration of contributing to open source and fixes an issue with a 404 link in the testing documentation.

[ijjk]

Stats from current PR

Default Build (Decrease detected ✓)

General Overall decrease ✓

	vercel/next.js canary	vercel/next.js fix-33406	Change
buildDuration	15.8s	15.6s	-215ms
buildDurationCached	3.5s	3.5s	⚠️ +31ms
nodeModulesSize	352 MB	352 MB	-7 B

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	vercel/next.js fix-33406	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.186	3.21	⚠️ +0.02
/ avg req/sec	784.68	778.82	⚠️ -5.86
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.439	1.469	⚠️ +0.03
/error-in-render avg req/sec	1737.58	1702.2	⚠️ -35.38

Client Bundles (main, webpack, commons)

	vercel/next.js canary	vercel/next.js fix-33406	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.2 kB	42.2 kB	✓
main-HASH.js gzip	27.2 kB	27.2 kB	✓
webpack-HASH.js gzip	1.44 kB	1.44 kB	✓
Overall change	71 kB	71 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js fix-33406	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js fix-33406	Change
_app-HASH.js gzip	1.37 kB	1.37 kB	✓
_error-HASH.js gzip	194 B	194 B	✓
amp-HASH.js gzip	312 B	312 B	✓
css-HASH.js gzip	326 B	326 B	✓
dynamic-HASH.js gzip	2.37 kB	2.37 kB	✓
head-HASH.js gzip	350 B	350 B	✓
hooks-HASH.js gzip	919 B	919 B	✓
image-HASH.js gzip	4.7 kB	4.7 kB	✓
index-HASH.js gzip	263 B	263 B	✓
link-HASH.js gzip	2.13 kB	2.13 kB	✓
routerDirect..HASH.js gzip	321 B	321 B	✓
script-HASH.js gzip	383 B	383 B	✓
withRouter-HASH.js gzip	318 B	318 B	✓
85e02e95b279..7e3.css gzip	107 B	107 B	✓
Overall change	14.1 kB	14.1 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js fix-33406	Change
_buildManifest.js gzip	459 B	459 B	✓
Overall change	459 B	459 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js fix-33406	Change
index.html gzip	531 B	531 B	✓
link.html gzip	545 B	545 B	✓
withRouter.html gzip	526 B	526 B	✓
Overall change	1.6 kB	1.6 kB	✓

---

Default Build with SWC (Increase detected ⚠️)

General Overall decrease ✓

	vercel/next.js canary	vercel/next.js fix-33406	Change
buildDuration	17.1s	17.2s	⚠️ +23ms
buildDurationCached	3.5s	3.5s	⚠️ +4ms
nodeModulesSize	352 MB	352 MB	-7 B

Page Load Tests Overall increase ✓

	vercel/next.js canary	vercel/next.js fix-33406	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.246	3.188	-0.06
/ avg req/sec	770.3	784.23	+13.93
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.487	1.423	-0.06
/error-in-render avg req/sec	1681.14	1756.72	+75.58

Client Bundles (main, webpack, commons)

	vercel/next.js canary	vercel/next.js fix-33406	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.3 kB	42.3 kB	✓
main-HASH.js gzip	27.3 kB	27.3 kB	✓
webpack-HASH.js gzip	1.44 kB	1.44 kB	✓
Overall change	71.3 kB	71.3 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js fix-33406	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js fix-33406	Change
_app-HASH.js gzip	1.35 kB	1.35 kB	✓
_error-HASH.js gzip	180 B	180 B	✓
amp-HASH.js gzip	305 B	305 B	✓
css-HASH.js gzip	321 B	321 B	✓
dynamic-HASH.js gzip	2.36 kB	2.36 kB	✓
head-HASH.js gzip	342 B	342 B	✓
hooks-HASH.js gzip	911 B	911 B	✓
image-HASH.js gzip	4.73 kB	4.73 kB	✓
index-HASH.js gzip	256 B	256 B	✓
link-HASH.js gzip	2.19 kB	2.19 kB	✓
routerDirect..HASH.js gzip	314 B	314 B	✓
script-HASH.js gzip	375 B	375 B	✓
withRouter-HASH.js gzip	309 B	309 B	✓
85e02e95b279..7e3.css gzip	107 B	107 B	✓
Overall change	14.1 kB	14.1 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js fix-33406	Change
_buildManifest.js gzip	459 B	459 B	✓
Overall change	459 B	459 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js fix-33406	Change
index.html gzip	531 B	531 B	✓
link.html gzip	545 B	545 B	✓
withRouter.html gzip	526 B	526 B	✓
Overall change	1.6 kB	1.6 kB	✓

Commit: a686e84