docs: Fix link to org_agenda in custom commands docs

Author: kristijanhusak

docs/configuration.org

@@ -596,7 +596,7 @@ agenda will always show current week from today - 2 days
 - Default: ={}=
 
 Define custom agenda views that are available through the
-(org_agenda)[#org_agenda] mapping. It is possible to combine multiple
+[[#org_agenda][org_agenda]] mapping. It is possible to combine multiple
 agenda types into single view. An example:
 
 #+begin_src lua