Fix misspellings of "invocation" in comments.

Some comments misspell "invocation"; this fixes them. No code
changes.

Signed-off-by: Adam Buchbinder <adam.buchbinder@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 30fe668..e60a340 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -349,7 +349,7 @@
 	 * prev_priority holds the scanning priority for this zone.  It is
 	 * defined as the scanning priority at which we achieved our reclaim
 	 * target at the previous try_to_free_pages() or balance_pgdat()
-	 * invokation.
+	 * invocation.
 	 *
 	 * We use prev_priority as a measure of how much stress page reclaim is
 	 * under - it drives the swappiness decision: whether to unmap mapped