Update verify.rst

This is my first contribution to Linux Mint documentation.

I found this tidbit on Stackoverflow.com after searching Reddit. Hopefully it can help some of the many Windows users with switching.

https://stackoverflow.com/questions/478722/what-is-the-best-way-to-calculate-a-checksum-for-a-file-that-is-on-my-machine

https://www.reddit.com/r/sysadmin/comments/383ry7/til_windows_can_actually_natively_do_checksums/