doc/site/build-ref.html: fix export error
Fixes
@@ -201,6 +201,12 @@ src/my/app/tests/test.cc
this label is usually written as
</p>
+<pre>
+//my/app/main:app_binary
+</pre>
+
+<p>
+
Each label has two parts, a package name (<code>my/app/main</code>)
and a target name (<code>app_binary</code>). Every label uniquely
identifies a target. Labels sometimes appear in other forms; when
