proof: don't confuse ""(1) (thanks Stuart Morrow)

"" looks for patterns in the form 'prompt;' or 'prompt%',
and gets confused when proof emits 'illegal;'. This change
replaces the ';' with a ':', which both matches other
conventional error outputs and prevents "" from getting
confused.

sys/src/cmd/proof/htroff.c

@@ -550,7 +550,7 @@ botpage(int np)	/* called at bottom of page np-1 == top of page np */
 			continue;
 		}
 
-		fprint(2, "illegal;  try q, 17, +2, -1, p, m.7, /2, x1, y-.5 or return\n");
+		fprint(2, "illegal:  try q, 17, +2, -1, p, m.7, /2, x1, y-.5 or return\n");
 	}
 	return 0;
 }