doc: add line break in install-source.html

Change-Id: I8633a005da7d0c34a5f074fbd979c8e0fc1fba37
Reviewed-on: https://go-review.googlesource.com/12505Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/install-source.html

@@ -487,6 +487,7 @@ The valid combinations of <code>$GOOS</code> and <code>$GOARCH</code> are:
 <td></td><td><code>windows</code></td> <td><code>amd64</code></td>
 </tr>
 </table>
+<br>
 
 <li><code>$GOHOSTOS</code> and <code>$GOHOSTARCH</code>
 <p>