doc: Fix whitespace in syntax.

Back-patch to 10.

Author: Andreas Karlsson
Discussion: https://postgr.es/m/043acae2-a369-b7fa-be48-1933aa2e82d1%40proxel.se

doc/src/sgml/ref/insert.sgml

@@ -23,7 +23,7 @@ PostgreSQL documentation
 <synopsis>
 [ WITH [ RECURSIVE ] <replaceable class="parameter">with_query</replaceable> [, ...] ]
 INSERT INTO <replaceable class="parameter">table_name</replaceable> [ AS <replaceable class="parameter">alias</replaceable> ] [ ( <replaceable class="parameter">column_name</replaceable> [, ...] ) ]
-    [ OVERRIDING { SYSTEM | USER} VALUE ]
+    [ OVERRIDING { SYSTEM | USER } VALUE ]
     { DEFAULT VALUES | VALUES ( { <replaceable class="parameter">expression</replaceable> | DEFAULT } [, ...] ) [, ...] | <replaceable class="parameter">query</replaceable> }
     [ ON CONFLICT [ <replaceable class="parameter">conflict_target</replaceable> ] <replaceable class="parameter">conflict_action</replaceable> ]
     [ RETURNING * | <replaceable class="parameter">output_expression</replaceable> [ [ AS ] <replaceable class="parameter">output_name</replaceable> ] [, ...] ]