Update to the MogileFS setup guide

I've done minor changes to  the installaion guide for MogileFS following feedback from Craig.

In addition, he noticed that on some Xen VMs, you may encounter the following error:

ERROR: Need to be root to increase max connections

in which case, you will need to update /etc/security/limits.conf with the values:

mogile soft nofile 65535
mogile hard nofile 65535