Fix bug 1400331: Rename WebExtensions link in main nav.

[wbamberg]

I've tested this in a local Kuma instance and it looks all right.

One thing I wasn't sure about was case: I've used "Browser Extensions" here, to match the name in the standard: https://browserext.github.io/browserext/, but the title of the linked page is actually "Browser extensions". I think probably the title of the page should be changed to match the name of the standard.

[codecov-io]

Codecov Report

Merging #4428 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4428   +/-   ##
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  Coverage   94.74%   94.74%           
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  Files         257      257           
  Lines       22695    22695           
  Branches     1670     1670           
=======================================
  Hits        21502    21502           
  Misses        964      964           
  Partials      229      229

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[stephaniehobson]

👍

And it's a good idea to change the page title to match.