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[Extensions docs] Update browseraction.json docs

Update browseraction.json docs for minor language and formatting

Bug: none
Change-Id: Id7f5f33bbd4193ed107d2e10da612d32f13a701b
Reviewed-on: https://chromium-review.googlesource.com/1217703
Commit-Queue: David Gash <dgash@google.com>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595550}