Fix typo in nlfor() docstring

dotfiles/bash/main.bashrc

     #     /bar
     #     $
     #
-    # is almost like `tr -n :` except that it is a bit easier
+    # is almost like `tr : '\n'` except that it is a bit easier
     # to type and fixes the annoying problem with missing
     # newline at the end of the replaced string.
     #