Fix missing space in docs.

Author: Japin Li
Discussion: https://www.postgresql.org/message-id/MEYP282MB1669C36E5F733C2EFBDCB80BB6619@MEYP282MB1669.AUSP282.PROD.OUTLOOK.COM

doc/src/sgml/arch-dev.sgml

@@ -559,7 +559,7 @@
     A simple <command>INSERT ... VALUES</command> command creates a
     trivial plan tree consisting of a single <literal>Result</literal>
     node, which computes just one result row, feeding that up
-    to<literal>ModifyTable</literal> to perform the insertion.
+    to <literal>ModifyTable</literal> to perform the insertion.
    </para>
 
   </sect1>