Fix description how to create conversion function.

Author: tatsuo-ishii

src/backend/utils/mb/conversion_procs/README.euc_jp

@@ -1,6 +1,6 @@
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-	2002/09/18 Tatsuo Ishii
+	2006/04/15 Tatsuo Ishii
 
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@@ -24,7 +24,7 @@ o C
 	INTEGER,	-- source encoding id
 	INTEGER,	-- destination encoding id
 	CSTRING,	-- source string (null terminated C string)
-	CSTRING,	-- destination string (null terminated C string)
+	INTERNAL,	-- destination string (null terminated C string)
 	INTEGER		-- source string length
   ) returns VOID;