fix #2098 and a typo

fix #2098 and a typo

The phrasing regarding copying the templates from '/var/lib/pmg/templates'
to '/etc/pmg/templates' caused some confusion [0] and some users thought they
need to copy all templates to '/etc/pmg/templates', even if they only modified
one template. Rephrasing the sentence to speak of a singular template, should
explain the workings better.

Additionally a singular/plural typo was fixed

[0] https://bugzilla.proxmox.com/show_bug.cgi?id=2098 and a few forum-threads

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>