s/source tree/repository/

The top of the repository has been well-defined; the notion of a “source tree” hasn't.

Closes bazelbuild#13038.

PiperOrigin-RevId: 358758438

site/docs/build-ref.html

@@ -335,8 +335,8 @@ <h4>Package names, <code>//<b>package-name</b>:...</code></h4>
 <p>
   The name of a package is the name of the directory containing its
 
-  BUILD file, relative to the top-level directory of the source tree.
-  For example: <code>my/app</code>.
+  BUILD file, relative to the top-level directory of the containing
+  repository. For example: <code>my/app</code>.
 
   Package names must be composed entirely of characters drawn from
   the set <code>A</code>-<code>Z</code>, <code>a</code>–<code>z</code>,