static_keys.txt: Fix trivial spelling mistake

Fix the spelling of 'functionnality' -> 'functionality'.
Signed-off-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/static-keys.txt

@@ -159,7 +159,7 @@ particularly the CPU hotplug lock (in order to avoid races against
 CPUs being brought in the kernel while the kernel is getting
 patched). Calling the static key API from within a hotplug notifier is
 thus a sure deadlock recipe. In order to still allow use of the
-functionnality, the following functions are provided:
+functionality, the following functions are provided:
 
 	static_key_enable_cpuslocked()
 	static_key_disable_cpuslocked()