selftests/vm/pkeys: fix typo in comment

Spelling mistake (triple letters) in comment. Detected with the help of
Coccinelle.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Julia Lawall <[e-mail]>
Reviewed-by: Muchun Song <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

tools/testing/selftests/vm/protection_keys.c

···
 	/*
 	 * Reset the shadow, assuming that the above mprotect()
 	 * correctly changed PKRU, but to an unknown value since
-	 * the actual alllocated pkey is unknown.
+	 * the actual allocated pkey is unknown.
 	 */
 	shadow_pkey_reg = __read_pkey_reg();