doc: Fix reference to "decoder" to instead be the correct "output plugin".

Author: Jonathan Katz
Discussion: https://postgr.es/m/DD02DD86-5989-4BFD-8712-468541F68383@postgresql.org
Backpatch: 9.4-, where logical decoding was added

doc/src/sgml/test-decoding.sgml

@@ -10,7 +10,7 @@
  <para>
   <filename>test_decoding</filename> is an example of a logical decoding
   output plugin. It doesn't do anything especially useful, but can serve as
-  a starting point for developing your own decoder.
+  a starting point for developing your own output plugin.
  </para>
 
  <para>