Add the output file to the usage

scripts/maint/updateFallbackDirs.py

@@ -1,7 +1,8 @@
 #!/usr/bin/python
 
-# Usage: scripts/maint/updateFallbackDirs.py > src/or/fallback_dirs.inc
-#        scripts/maint/updateFallbackDirs.py check_existing
+# Usage:
+# scripts/maint/updateFallbackDirs.py > src/or/fallback_dirs.inc
+# scripts/maint/updateFallbackDirs.py check_existing > src/or/fallback_dirs.inc
 #
 # This script should be run from a stable, reliable network connection,
 # with no other network activity (and not over tor).