Fix spelling error.

fdtextract.c

@@ -49,7 +49,7 @@ static struct option const usage_long_opts[] = {
 static const char * const usage_opts_help[] = {
     "List images embedded in FIT",
     "Extract image from FIT",
-    "Outpute image name",
+    "Output image name",
     USAGE_COMMON_OPTS_HELP
 };