AW docs: fix link to writable system instructions

The link to the android_emulator page should have been an absolute link,
not a relative one.

Bug: 918221
Test: tools/md_browser/md_browser.py android_webview/docs/webview-shell.md
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I6bae38ef2b111534604beae2442fc28e93038f9b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1913616
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Shimi Zhang <ctzsm@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715549}

android_webview/docs/webview-shell.md

@@ -49,7 +49,7 @@ If this occurs then delete the preinstalled WebView shell as so:
 *** note
 **Note:** If using the emulator ensure it is being started with the
 `-writable-system` option as per the
-[Writable system partition](docs/android_emulator.md#writable-system-partition)
+[Writable system partition](/docs/android_emulator.md#writable-system-partition)
 instructions.
 ***