typo fix

Author: splitbrain

grapher.php

@@ -273,7 +273,7 @@ function usage(){
  OPTIONS
  -h, --help show this help and exit
  -d, --depth <num> recursion depth. 0 for all. default: 1
- -f, --format (dot|gfx)  output format, default: dot
+ -f, --format (dot|gexf) output format, default: dot
  -m, --media (ns|all|none) how to handle media files. default: ns
  -o, --output <file> where to store the output. default: STDOUT
 ";