Make `ambush` username matching non-greedy

Previously messages such as this one would not work:

  ambush Benjie: Hey did you see this link: http://google.com/

because the script would think the username was:
`Benjie: Hey did you see this link`
rather than `Benjie` as expected.

This single-character change matches the first `: ` rather than the
last, thereby solving this problem.

(Only breakage would be for users who have `: ` in their username,
which seems more of an edge case than having `: ` in the message
body.)