Make `ambush` username matching non-greedy

Previously messages such as this one would not work:

    ambush Benjie: Hey did you see this link: http://example.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.)