[CentOS-docs] Improvement suggestion to "How do I disable IPv6"

[Ville Skyttä]

http://wiki.centos.org/FAQ/CentOS5#head-47912ebdae3b5ac10ff76053ef057c366b421dc4

This tip suggests creating a /etc/modprobe.d/disable-ipv6 file.  However 
modprobe from module-init-tools >= 3.7 will output this warning for it:

| WARNING: All config files need .conf: /etc/modprobe.d/disable-ipv6, it will
| be ignored in a future release.

Simply changing the recipe to create a "disable-ipv6.conf" file instead of 
"disable-ipv6" would make it more future proof.