Correct class name formatting in locking.md (#8316)

* Correct class name formatting in locking.md

* Escape namespace separators and keep class name bold

A single backslash before `L` was treated as a Markdown escape, eating the
`L` and breaking the header. Escape the namespace separators with double
backslashes and keep the short class name bold, matching every other page
(e.g. caching.md, views.md).

---------

Co-authored-by: Mark Scherer <dereuromark@users.noreply.github.com>

Author: PhantomWatson

docs/en/core-libraries/locking.md

@@ -1,6 +1,6 @@
 # Locking
 
-`class` Cake\Lock\**Lock**
+`class` Cake\\Lock\\**Lock**
 
 Locking helps you coordinate access to shared resources across concurrent
 requests, CLI commands, queue workers, or background jobs. Use locks when you