correcting return value when destination key already exists for copy command (#2415)

As per the current design of copy commands it returns zero when destination already exist.
this is also documented in the `Returns` section below this text.

Author: hwware

commands/copy.md

@@ -5,7 +5,7 @@ By default, the `destination` key is created in the logical database used by the
 connection. The `DB` option allows specifying an alternative logical database
 index for the destination key.
 
-The command returns an error when the `destination` key already exists. The
+The command returns zero when the `destination` key already exists. The
 `REPLACE` option removes the `destination` key before copying the value to it.
 
 @return
@@ -21,4 +21,4 @@ The command returns an error when the `destination` key already exists. The
 SET dolly "sheep"
 COPY dolly clone
 GET clone
-```
+```