Fix stray sentence fragment in shared_preload_libraries documentation

The introduction in the Shared Library Preloading section already
instructs the user to separate multiple library names with commas, so
just remove the fragment from here.

Author: Dagfinn Ilmari Mannsåker

doc/src/sgml/config.sgml

@@ -6416,7 +6416,7 @@ SET XML OPTION { DOCUMENT | CONTENT };
       <listitem>
        <para>
         This variable specifies one or more shared libraries to be preloaded at
-        server start.  with commas.  This parameter can only be set at server
+        server start.  This parameter can only be set at server
         start.  If a specified library is not found, the server will fail to
         start.
        </para>