Change handbook to use c7 instead of c6 with upcoming deprecation

[debdrup]

Summary

Since the linux emulation in FreeBSD based on CentOS 6 being deprecated - according to the handbook on the day year of 2019, ie. the 31st - it's probably a good idea to replace the examples used in the handbook with ones based on CentOS 7.

Test Plan

Since it's a simple regex, is there a need for testing?

[rene]

I indeed plan to remove the Centos 6 on the 31st. Patch looks good to me.

[debdrup]

Feel free to commit at your leisure then - since I still don't have svn access, just project membership (that I need to get setup).