Add missing space

It is required, otherwise this looks like an anonymous link to the
parser, which then thinks the contents of the backticks is supposed to
be the path to a document.

docs/rst-reference/text-roles.rst

@@ -6,7 +6,7 @@ Text Roles
 
 Text roles can be used to style content inline. Some text roles have advanced processing such as reference resolving.
 
-You can also :doc:`add your own custom text roles</extension/text-roles>`.
+You can also :doc:`add your own custom text roles </extension/text-roles>`.
 
 Currently the following text roles are implemented: