[documentation] fix link to preview of viewport addon

[Kiwka]

Issue:

Preview of viewport addon has broken link in documentation for addon

What I did

Fixed link to preview

How to test

Visually in Readme.md file

[Hypnosphi]

Thanks @Kiwka

@ndelangen shouldn't we use the approach from #2644 for this image as well?

[codecov]

Codecov Report

Merging #2696 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2696   +/-   ##
=======================================
  Coverage   34.35%   34.35%           
=======================================
  Files         389      389           
  Lines        8747     8747           
  Branches      909      906    -3     
=======================================
  Hits         3005     3005           
+ Misses       5143     5131   -12     
- Partials      599      611   +12

Impacted Files	Coverage Δ
app/vue/src/server/utils.js	0% <0%> (-53.58%)	⬇️
lib/ui/src/modules/api/actions/api.js	51.85% <0%> (ø)	⬆️
addons/knobs/src/components/types/Array.js	34.14% <0%> (ø)	⬆️
lib/ui/src/modules/ui/configs/handle_keyevents.js	33.33% <0%> (ø)	⬆️
...ddons/actions/src/components/ActionLogger/index.js	74.35% <0%> (ø)	⬆️
addons/knobs/src/components/types/Boolean.js	11.62% <0%> (ø)	⬆️
app/react/src/client/preview/error_display.js	54.54% <0%> (ø)	⬆️
lib/components/src/table/table.js	83.33% <0%> (ø)	⬆️
lib/ui/src/modules/ui/components/layout/index.js	29.28% <0%> (ø)	⬆️
...-native/src/preview/components/OnDeviceUI/index.js	0% <0%> (ø)	⬆️
... and 43 more

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