doc: update rcu_dereference.rst reference

Changeset b00aedf9 ("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.

Fixes: b00aedf9 ("doc: Convert to rcu_dereference.txt to rcu_dereference.rst")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>