Doc: update recovery/README.

Commit e2f65f4 added contrib/pg_prewarm to the prerequisites for
running the src/test/recovery suite, but did not bother to update
the documentation about that.

Author: tglsfdc

src/test/recovery/README

@@ -10,7 +10,7 @@ Running the tests
 
 NOTE: You must have given the --enable-tap-tests argument to configure.
 Also, to use "make installcheck", you must have built and installed
-contrib/test_decoding in addition to the core code.
+contrib/pg_prewarm and contrib/test_decoding in addition to the core code.
 
 Run
     make check