Update INSTALL.txt

[kinow]

Running these commands worked for me on an Ubuntu 16.04. Never used Perl on this computer, so probably that's why I didn't have Net::Netmask.

With only String::Random, it complained about the two other missing modules.

After that, everything works fine (not sure about Mac though). Thanks a lot for dnsenum!

Oh, fixed a typo as well (should have branched the changes, sorry for using master, had short time to submit the PR).