replace artificial space with RDoc markup (follow up #11285). [ci skip]

activerecord/lib/active_record/validations/uniqueness.rb

@@ -197,8 +197,8 @@ module ClassMethods
       # will result in the default Rails exception page being shown), or you
       # can catch it and restart the transaction (e.g. by telling the user
       # that the title already exists, and asking him to re-enter the title).
-      # This technique is also known as optimistic concurrency control:
-      # http://en.wikipedia.org/wiki/Optimistic_concurrency_control .
+      # This technique is also known as
+      # {optimistic concurrency control}[http://en.wikipedia.org/wiki/Optimistic_concurrency_control].
       #
       # The bundled ActiveRecord::ConnectionAdapters distinguish unique index
       # constraint errors from other types of database errors by throwing an