Correct documentation about starting code-review

The button says "START REVIEW" these days.

Change-Id: Ia285ce8b0b7f8d6698f31e070c60e99f8ffacb8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2043814
Auto-Submit: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739406}

docs/contributing.md

@@ -233,8 +233,8 @@ list of the reviewers you picked.
 
 In the same dialog, you can include an optional message to your reviewers. This
 space can be used for specific questions or instructions. Once you're done,
-make sure to click **Send**, which notifies the requested reviewers that they
-should review your change.
+make sure to click **Start Review**, which notifies the requested reviewers that
+they should review your change.
 
 **IMPORTANT: UNTIL YOU SEND THE REVIEW REQUEST, NO ONE WILL LOOK AT THE REVIEW**