fix JSON deep copy description (#23495)

Hi,

This is a tiny change, fixing the error in the documentation of JSON's
deep copy proc.

lib/pure/json.nim

@@ -634,7 +634,7 @@ proc delete*(obj: JsonNode, key: string) =
  obj.fields.del(key)
 
 proc copy*(p: JsonNode): JsonNode =
- ## Performs a deep copy of `a`.
+ ## Performs a deep copy of `p`.
  case p.kind
  of JString:
  result = newJString(p.str)