doc: Update rcu_dereference.rst reference

Changeset b00aedf978aa ("doc: Convert to rcu_dereference.txt to rcu_dereference.rst")
renamed: Documentation/RCU/rcu_dereference.txt
to: Documentation/RCU/rcu_dereference.rst.

Update its cross-reference accordingly.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>

tools/memory-model/Documentation/glossary.txt

···
 	 from the value returned by the rcu_dereference() on line 2, the
 	 address dependency extends from that rcu_dereference() to that
 	 "p->a".  In rare cases, optimizing compilers can destroy address
-	 dependencies.	Please see Documentation/RCU/rcu_dereference.txt
+	 dependencies.	Please see Documentation/RCU/rcu_dereference.rst
 	 for more information.
 
 	 See also "Control Dependency" and "Data Dependency".