Updated help text to reflect new options.

SpritePacker/Resources/HelpText.txt

@@ -2,7 +2,7 @@
 Built at {build-date}
 
 Usage:
-    mono ./SpritePacker.exe [flags] [filenames]
+    mono ./SpritePacker.exe [flags] /path/to/output/file.png [filenames]
 
 --help		Shows this help message.
 --version	Shows the version information.