Filename change to wpt_internal README file

Small documentation fixes for wpt_internal, file name change from README to README.md for readability.

Change-Id: I8043180bd6469c5b6be75f70acb8ca30113471b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3149809
Reviewed-by: Xianzhu Wang <[email protected]>
Commit-Queue: Shunya Shishido <[email protected]>
Cr-Commit-Position: refs/heads/main@{#920430}
diff --git a/docs/testing/web_platform_tests.md b/docs/testing/web_platform_tests.md
index 29aad9b..334e447 100644
--- a/docs/testing/web_platform_tests.md
+++ b/docs/testing/web_platform_tests.md
@@ -134,7 +134,7 @@
 upstreamed to WPT.
 
 Please see the `wpt_internal`
-[README](../../third_party/blink/web_tests/wpt_internal/README) for more details.
+[README](../../third_party/blink/web_tests/wpt_internal/README.md) for more details.
 
 **Note**: A significant downside of `wpt_internal` is that your tests may be
 broken by upstream changes to the resources scripts (e.g. `testharness.js`), as