Fix broken doc link. [skip ci]

docs/installing_from_source.rst

@@ -16,7 +16,7 @@ General Notes
   install --no-binary gevent gevent``. This is useful if there is a
   binary wheel available, but you want to change the compile-time
   options, such as to use a system version of libuv instead of the
-  embedded version. See ``
+  embedded version. See :ref:`build-config`.
 
 - You'll need a working C compiler that can build Python extensions.
   On some platforms, you may need to install Python development
@@ -81,6 +81,8 @@ those compiling gevent from source.
   platform_python_implementation == 'CPython'")``, the version of
   setuptools is too old. Install a more recent version of setuptools.
 
+.. _build-config:
+
 Build-Time Configuration
 ========================