[CF 5916] small fix for pg_overexplain docs

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Patch(es): https://www.postgresql.org/message-id/aHVWKM8l8kLlZzgv@nathan
Author(s): Nathan Bossart

Author: Commitfest Bot

doc/src/sgml/pgoverexplain.sgml

@@ -8,7 +8,7 @@
  </indexterm>
 
  <para>
-  The <filename>pg_overexplain</filename> extends <command>EXPLAIN</command>
+  <filename>pg_overexplain</filename> extends <command>EXPLAIN</command>
   with new options that provide additional output. It is mostly intended to
   assist with debugging of and development of the planner, rather than for
   general use. Since this module displays internal details of planner data