doc: add android/arm as a valid GOOS/GOARCH combination

Fixes #14497.

Change-Id: Ibdd55acf9e416873c64f8751c2f65f7ccdb1d500
Reviewed-on: https://go-review.googlesource.com/19914Reviewed-by: David Crawshaw <crawshaw@golang.org>

doc/install-source.html

@@ -445,6 +445,9 @@ The valid combinations of <code>$GOOS</code> and <code>$GOARCH</code> are:
 <th width="50"></th><th align="left" width="100"><code>$GOOS</code></th> <th align="left" width="100"><code>$GOARCH</code></th>
 </tr>
 <tr>
+<td></td><td><code>android</code></td> <td><code>arm</code></td>
+</tr>
+<tr>
 <td></td><td><code>darwin</code></td> <td><code>386</code></td>
 </tr>
 <tr>