Correct docs about allowed syntax of exports_filter

`//foo:__pkg__`, but not `//foo:__package__`.

src/main/starlark/builtins_bzl/common/cc/cc_shared_library.bzl

@@ -1029,7 +1029,7 @@ symbols should be exported.
 </p>
 
 <p>The following syntax is allowed:</p>
-<p><code>//foo:__package__</code> to account for any target in foo/BUILD</p>
+<p><code>//foo:__pkg__</code> to account for any target in foo/BUILD</p>
 <p><code>//foo:__subpackages__</code> to account for any target in foo/BUILD or any other
 package below foo/ like foo/bar/BUILD</p>"""),
         "win_def_file": attr.label(allow_single_file = [".def"], doc = """