Fix broken tag to external.html
The link to the [Working with External Dependencies](https://docs.bazel.build/versions/master/external.html) page in the `#specifying-targets-to-build` section  is broken due to missing `<` opening the `a` tag.
Closes
@@ -160,7 +160,7 @@ workspace.
<tr>
<td><code>//...</code></td>
<td>All targets in packages in the workspace. This does not include targets
- from a href="external.html">external repositories</a>.</td>
+ from <a href="external.html">external repositories</a>.</td>
</tr>
<tr>
<td><code>//:all</code></td>
