nfs.man: Fix macro use for fstab examples

The groff macros for filling (word-wrapping) and tabulation control are
lower-case, but are written in upper-case here and so have been ignored.

Change the .NF and .FI lines to lower-case.

Change the .TA lines to lower-case and fix the tab stops to work both
on a terminal and in Postscript output.

Delete the .SP line where .sp would be redundant.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Luk Claes <luk@debian.org>
Signed-off-by: Steve Dickson <steved@redhat.com>