encoding/json: update docs to not use misuse the term "object"

In JSON terminology, "object" is a collect of key/value pairs. But a
JSON object is only one type of JSON value (others are string, number,
array, true, false, null).

This updates the Go docs (at least the public godoc) to not use
"object" when we mean any JSON value.

Change-Id: Ieb1c456c703693714d63d9d09d306f4d9e8f4597
Reviewed-on: https://go-review.googlesource.com/22003
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/encoding/json/decode.go

@@ -97,7 +97,7 @@ func Unmarshal(data []byte, v interface{}) error {
 	return d.unmarshal(v)
 }
 
-// Unmarshaler is the interface implemented by objects
+// Unmarshaler is the interface implemented by types
 // that can unmarshal a JSON description of themselves.
 // The input can be assumed to be a valid encoding of
 // a JSON value. UnmarshalJSON must copy the JSON data