doc: Remove curly quotes from shell examples in man pages

Use \(aq rather than a literal ' as groff will convert apostrophe
to a right quote (\u2029) in utf8 locales for example.
Stepan Kasal details the issue and fix here:
http://lists.gnu.org/archive/html/bug-coreutils/2008-12/msg00124.html

man/du.x

@@ -19,7 +19,7 @@ will match any files whose names end in
 .BR .o .
 Therefore, the command
 .IP
-.B du --exclude='*.o'
+.B du --exclude=\(aq*.o\(aq
 .PP
 will skip all files and subdirectories ending in
 .BR .o

man/help2man

@@ -567,7 +567,7 @@ and
 .B %s
 programs are properly installed at your site, the command
 .IP
-.B info coreutils '%s invocation'
+.B info coreutils \(aq%s invocation\(aq
 .PP
 should give you access to the complete manual.
 EOT