doc: Improve COPY synopsis

Upper-case MATCH so that it is clearer that it is a keyword in this
context.

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> | match ]
+    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> [, ...] ) | * }
@@ -278,7 +278,7 @@ COPY { <replaceable class="parameter">table_name</replaceable> [ ( <replaceable
       column in the file.  On output, the first line contains the column
       names from the table.  On input, the first line is discarded when this
       option is set to <literal>true</literal> (or equivalent Boolean value).
-      If this option is set to <literal>match</literal>, the number and names
+      If this option is set to <literal>MATCH</literal>, the number and names
       of the columns in the header line must match the actual column names of
       the table, otherwise an error is raised.
       This option is not allowed when using <literal>binary</literal> format.