perldiag: Add some double quotes

There are two areas of text, supposedly identical, and used because of
splain.  They had drifted apart slightly, so this brings them back to
consistency.

(cherry picked from commit 21792e6)

Author: khwilliamson

pod/perldiag.pod

@@ -6831,24 +6831,24 @@ Check the #! line, or manually feed your script into Perl yourself.
 =item Unescaped left brace in regex is deprecated here (and will be fatal in Perl 5.32), passed through in regex; marked by S<<-- HERE> in m/%s/
 
 (D deprecated, regexp)  The simple rule to remember, if you want to
-match a literal C<{> character (U+007B C<LEFT CURLY BRACKET>) in a
+match a literal C<"{"> character (U+007B C<LEFT CURLY BRACKET>) in a
 regular expression pattern, is to escape each literal instance of it in
 some way.  Generally easiest is to precede it with a backslash, like
-C<\{> or enclose it in square brackets (C<[{]>).  If the pattern
-delimiters are also braces, any matching right brace (C<}>) should
+C<"\{"> or enclose it in square brackets (C<"[{]">).  If the pattern
+delimiters are also braces, any matching right brace (C<"}">) should
 also be escaped to avoid confusing the parser, for example,
 
  qr{abc\{def\}ghi}
 
-Forcing literal C<{> characters to be escaped will enable the Perl
+Forcing literal C<"{"> characters to be escaped will enable the Perl
 language to be extended in various ways in future releases.  To avoid
 needlessly breaking existing code, the restriction is is not enforced in
 contexts where there are unlikely to ever be extensions that could
-conflict with the use there of C<{> as a literal.
+conflict with the use there of C<"{"> as a literal.
 
-In this release of Perl, some literal uses of C<{> are fatal, and some
+In this release of Perl, some literal uses of C<"{"> are fatal, and some
 still just deprecated.  This is because of an oversight:  some uses of a
-literal C<{> that should have raised a deprecation warning starting in
+literal C<"{"> that should have raised a deprecation warning starting in
 v5.20 did not warn until v5.26.  By making the already-warned uses fatal
 now, some of the planned extensions can be made to the language sooner.
 The cases which are still allowed will be fatal in Perl 5.30 or 5.32,
@@ -6861,12 +6861,12 @@ The contexts where no warnings or errors are raised are:
 
 =item *
 
-as the first character in a pattern, or following C<^> indicating to
+as the first character in a pattern, or following C<"^"> indicating to
 anchor the match to the beginning of a line.
 
 =item *
 
-as the first character following a C<|> indicating alternation.
+as the first character following a C<"|"> indicating alternation.
 
 =item *