Fix markup in synopsis

There is no need for a <literal> inside a <synopsis>, since the latter
is already all "literal" implicitly.  Also, create_help.pl misparses
it.  So just remove it.

Reported-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/20220517.174342.1884842412165214815.horikyota.ntt%40gmail.com

doc/src/sgml/ref/copy.sgml

@@ -36,7 +36,7 @@ COPY { <replaceable class="parameter">table_name</replaceable> [ ( <replaceable
     FREEZE [ <replaceable class="parameter">boolean</replaceable> ]
     DELIMITER '<replaceable class="parameter">delimiter_character</replaceable>'
     NULL '<replaceable class="parameter">null_string</replaceable>'
-    HEADER [ <replaceable class="parameter">boolean</replaceable> | <literal>match</literal> ]
+    HEADER [ <replaceable class="parameter">boolean</replaceable> | match ]
     QUOTE '<replaceable class="parameter">quote_character</replaceable>'
     ESCAPE '<replaceable class="parameter">escape_character</replaceable>'
     FORCE_QUOTE { ( <replaceable class="parameter">column_name</replaceable> [, ...] ) | * }