Fix extraneous parenthesis in WPT docs

Also edit a couple words.

Bug: 
Change-Id: I1398aac2be37aa2902a0def6892ad273946eb178
Reviewed-on: https://chromium-review.googlesource.com/752060
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Commit-Queue: Jeff Carpenter <jeffcarp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513629}

docs/testing/web_platform_tests.md

@@ -20,10 +20,8 @@ web-platform-tests, including tips for writing and reviewing tests.
 
 ## Importing tests
 
-Chromium has a mirror
-([web-platform-tests](https://chromium.googlesource.com/external/w3c/web-platform-tests/)
-of the
-GitHub repos, and periodically imports a subset of the tests so that they are
+Chromium has a [mirror](https://chromium.googlesource.com/external/w3c/web-platform-tests/)
+of the GitHub repo and periodically imports a subset of the tests to
 run as part of the regular Blink layout test testing process.
 
 The goals of this process are to be able to run web-platform-tests unmodified