Fix malformed link in using KUnit guide

Remove erroneous space causing a doc link to fail to render properly.

Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Change-Id: I8a44acf923ec049dfe073a85933ce6595b689068

usage/index.rst

diff --git a/usage/index.rst b/usage/index.rst
index cb3fd38..9a4aea7 100644
--- a/usage/index.rst
+++ b/usage/index.rst
@@ -15,7 +15,7 @@
 
 KUnit is integrated into the Linux kernel, so all you need is a version of the
 kernel which contains KUnit, and a config file with which to build it. The
-:doc: `getting_kunit` page covers the different versions of KUnit available, and
+:doc:`getting_kunit` page covers the different versions of KUnit available, and
 where to get them.