docs: Add missing colon

sphinx emits warning

       WARNING: Inline emphasis start-string without end-string.

This is caused by a missing colon.

Add missing colon, clearing shpinx build warning.
Signed-off-by: Tobin C. Harding <tobin@kernel.org>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/vm/slub.rst

@@ -66,7 +66,7 @@ Trying to find an issue in the dentry cache? Try::
 to only enable debugging on the dentry cache.  You may use an asterisk at the
 end of the slab name, in order to cover all slabs with the same prefix.  For
 example, here's how you can poison the dentry cache as well as all kmalloc
-slabs:
+slabs::
 
 	slub_debug=P,kmalloc-*,dentry