mount_nfs.8: Reword sentence so .Pa macro works

Commit 603677334a64 added a sentence with a file path
in it. However, it did not use .Pa since it would leave
a space after it, where ('s) was supposed to go.

This patch rewords the sentence so that .Pa can
be used.

This is a content change.

(cherry picked from commit f46bea05fea3faf0780a9e0fb4dbeb588e32a6ea)