docs: Fix typo in synchronize_rcu() function name

s/sychronize_rcu/synchronize_rcu/
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/RCU/whatisRCU.rst

@@ -360,7 +360,7 @@ order to amortize their overhead over many uses of the corresponding APIs.
 
 There are at least three flavors of RCU usage in the Linux kernel. The diagram
 above shows the most common one. On the updater side, the rcu_assign_pointer(),
-sychronize_rcu() and call_rcu() primitives used are the same for all three
+synchronize_rcu() and call_rcu() primitives used are the same for all three
 flavors. However for protection (on the reader side), the primitives used vary
 depending on the flavor: