find.1: fix rendering of find -s example

The period here is part of the literal string in the example.  Adding a
space caused the example to be quoted wrongly.  Instead, a trailing
zero-width space keeps the linter mandoc -T happy.

Reviewed by:	ziaee
Differential Revision:	https://reviews.freebsd.org/D59353

(cherry picked from commit 3581b6db62)

usr.bin/find/find.1

@@ -161,11 +161,11 @@ For example,
 puts a directory
 .Ql Ar foo
 with all its contents before a directory
-.Ql Ar foo .
+.Ql Ar foo.\&
 but
 .Ql "find | sort"
 puts the directory name
-.Ql Ar foo .
+.Ql Ar foo.\&
 before any string like
 .Ql Ar foo/bar
 because