(trivial) spelling fix in comment in Makefile

@@ -18,7 +18,7 @@
 #
 # Most importantly: sub-Makefiles should only ever modify files in
 # their own directory. If in some directory we have a dependency on
-# a file in another dir (which doesn't happen often, but it's of
+# a file in another dir (which doesn't happen often, but it's often
 # unavoidable when linking the built-in.o targets which finally
 # turn into vmlinux), we will call a sub make in that other dir, and
 # after that we are sure that everything which is in that other dir