fix: regexp in config.js

[Means88]

Issue: #6179

What I did

Add escape to dot in regex.

How to test

• Is this testable with Jest or Chromatic screenshots?
  I don't know how to test it with Jest.
• Does this need a new example in the kitchen sink apps?
  No.
• Does this need an update to the documentation?
  Yes, and it has updated in this PR.

If your answer is yes to any of these, please make sure to include it in your PR.

[codecov]

Codecov Report

Merging #6180 into next will increase coverage by 0.1%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             next    #6180     +/-   ##
=========================================
+ Coverage   37.93%   38.04%   +0.1%     
=========================================
  Files         645      643      -2     
  Lines        9626     9420    -206     
  Branches      352     1344    +992     
=========================================
- Hits         3652     3584     -68     
+ Misses       5927     5268    -659     
- Partials       47      568    +521

Impacted Files	Coverage Δ
lib/ui/src/settings/shortcuts_page.js	0% <0%> (-25%)	⬇️
lib/ui/src/containers/panel.js	14.28% <0%> (-19.05%)	⬇️
lib/ui/src/containers/nav.js	11.76% <0%> (-4.91%)	⬇️
addons/notes/src/Panel.tsx	39.62% <0%> (-4.28%)	⬇️
app/polymer/src/client/index.js	0% <0%> (ø)	⬆️
app/html/standalone.js	0% <0%> (ø)	⬆️
lib/core/src/server/config.js	0% <0%> (ø)	⬆️
lib/core/src/server/cli/prod.js	0% <0%> (ø)	⬆️
addons/storysource/src/preview.js	0% <0%> (ø)	⬆️
addons/info/src/components/types/OneOf.js	60% <0%> (ø)	⬆️
... and 227 more

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[shilman]

Good catch -- LGTM!