spelling: s/usefull/useful/

Signed-off-by: Adrian Bunk <bunk@stusta.de>
diff --git a/Makefile b/Makefile
index 599e744..fb497ea 100644
--- a/Makefile
+++ b/Makefile
@@ -251,7 +251,7 @@
 # If it is set to "silent_", nothing wil be printed at all, since
 # the variable $(silent_cmd_cc_o_c) doesn't exist.
 #
-# A simple variant is to prefix commands with $(Q) - that's usefull
+# A simple variant is to prefix commands with $(Q) - that's useful
 # for commands that shall be hidden in non-verbose mode.
 #
 #	$(Q)ln $@ :<