Add colon to 'To run' lines

Or else add an ostomy :-).

Some README.md files have severe ulcerative colitis and will need a
colectomies but others are missing colons. These will be discovered and
fixed in time. In the meantime I've restored some functionality for
these poor README.md files by giving them an ostomy.

2012/tromp/README.md

@@ -10,7 +10,7 @@ John Tromp
 
     make tromp32	# On a 32-bit machine
 
-## To run
+## To run:
 
     ./tromp [-b]