webfsget: use correct name in usage
 [rsc] --rw-rw-r-- M 303009 rsc sys 1597 Mar  9 09:17 sys/src/cmd/webfsget.c
	/n/sourcesdump/2006/0309/plan9/sys/src/cmd/webfsget.c:18,24 - 
	/n/sourcesdump/2006/0310/plan9/sys/src/cmd/webfsget.c:18,24
	  void
	  usage(void)
	  {
	- 	fprint(2, "usage: webget [-b baseurl] [-m mtpt] [-p postbody] url\n");
	+ 	fprint(2, "usage: webfsget [-b baseurl] [-m mtpt] [-p postbody] url\n");
	  	exits("usage");
	  }