doc: Fix COPY ON_ERROR option syntax synopsis.

ON_ERROR option values don't require quotations, which was
inconsistent with the syntax synopsis in the documentation.

Oversight in b725b7e.

Author: Atsushi Torikoshi
Reviewed-by: Masahiko Sawada
Discussion: https://postgr.es/m/CAD21AoC%3Dn4xR3%2BKQiqodnfT9chSB62XwZqmMff39H%3Dx9DS4scQ%40mail.gmail.com

Author: MasahikoSawada

doc/src/sgml/ref/copy.sgml

@@ -43,7 +43,7 @@ COPY { <replaceable class="parameter">table_name</replaceable> [ ( <replaceable
     FORCE_QUOTE { ( <replaceable class="parameter">column_name</replaceable> [, ...] ) | * }
     FORCE_NOT_NULL { ( <replaceable class="parameter">column_name</replaceable> [, ...] ) | * }
     FORCE_NULL { ( <replaceable class="parameter">column_name</replaceable> [, ...] ) | * }
-    ON_ERROR '<replaceable class="parameter">error_action</replaceable>'
+    ON_ERROR <replaceable class="parameter">error_action</replaceable>
     ENCODING '<replaceable class="parameter">encoding_name</replaceable>'
     LOG_VERBOSITY <replaceable class="parameter">mode</replaceable>
 </synopsis>