configure.ac: fix error message when md5sum is missing

I tried to run autogen.sh on freshly cloned LO repo. It prints kind of
cryptic message to say that md5sum utility is missing:

    configure: error: no md5sum: found!

Too many colons. I removed one, now it says:

    configure: error: no md5sum found!

FYI: this message was printing when I used md5sum from Inferno OS
project. LO probably needs some options which Inferno version don't
support (--version at least).

Change-Id: Iafdffede3337039dc95bb3d7d767d8622333de2a
Reviewed-on: https://gerrit.libreoffice.org/11341
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>