AW docs: fix navbar

The gitiles navbar doesn't correctly render the "home" link. It's
unclear why this started happening recently, but this CL fixes the
issue by explicitly specifying the path instead of using the "[home]"
alias (which is already used to make the logo clickable).

Bug: 918221
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I4e3db63f43f2cbe5809c44d247fd293faecbdad9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1674846
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Paul Miller <paulmiller@chromium.org>
Reviewed-by: Paul Miller <paulmiller@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672189}

android_webview/docs/navbar.md

 # WebView docs (go/webview-docs)
 
-* [Home][home]
+* [Home](/android_webview/docs/README.md)
 * [Quick start](/android_webview/docs/quick-start.md)
 * [Device setup](/android_webview/docs/device-setup.md)
 * [Build instructions](/android_webview/docs/build-instructions.md)