Bump references of Centos 6 to Centos 7 in the Linux Emulation chapter.

The corresponding Centos 6 ports will be removed soon as they expire tomorrow.

Submitted by: debdrup, carlavilla
Reviewed by: bcr, rene
Differential Revision: https://reviews.freebsd.org/D22955
Differential Revision: https://reviews.freebsd.org/D22956