doc: fix ' and ` in PDF output of code

* doc/coreutils.texi: Set txicodequoteundirected and
txicodequotebacktick so that ' and ` in code examples appear
as-is, rather than being transliterated to ’ and ‘.  E.g., prefer
“... this is equivalent to ‘tr '\303\266' '\305\201'’ and ...” to
“... this is equivalent to ‘tr ’\303\266’ ’\305\201’’ and ...”
in PDF output.

doc/coreutils.texi

@@ -4,6 +4,8 @@
 @include version.texi
 @settitle GNU Coreutils @value{VERSION}
 @documentencoding UTF-8
+@set txicodequoteundirected
+@set txicodequotebacktick
 @allowcodebreaks false
 
 @c %**end of header