encoding/json/v2: fix typo in documentation about errors.AsType

CL 708495 mass migrated the stdlib to use errors.AsType.
It rewrote the documentation, but uses the non-pointer type
of SemanticError or SyntacticError, which is invalid since
the Error method is declared on the pointer receiver.

Also, call errors.AsType on a separate line for readability.

Change-Id: I2eaf59614e2b58aa4bc8669ab81ce64168c54f13
Reviewed-on: https://go-review.googlesource.com/c/go/+/714660
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Johan Brandhorst-Satzkorn <[email protected]>
Auto-Submit: Joseph Tsai <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: dsnet

src/encoding/json/jsontext/state.go

@@ -24,7 +24,8 @@ import (
 // The name of a duplicate JSON object member can be extracted as:
 //
 //	err := ...
-//	if serr, ok := errors.AsType[jsontext.SyntacticError](err); ok && serr.Err == jsontext.ErrDuplicateName {
+//	serr, ok := errors.AsType[*jsontext.SyntacticError](err)
+//	if ok && serr.Err == jsontext.ErrDuplicateName {
 //		ptr := serr.JSONPointer // JSON pointer to duplicate name
 //		name := ptr.LastToken() // duplicate name itself
 //		...

src/encoding/json/v2/errors.go

@@ -29,7 +29,8 @@ import (
 // The name of an unknown JSON object member can be extracted as:
 //
 //	err := ...
-//	if serr, ok := errors.AsType[json.SemanticError](err); ok && serr.Err == json.ErrUnknownName {
+//	serr, ok := errors.AsType[*json.SemanticError](err)
+//	if ok && serr.Err == json.ErrUnknownName {
 //		ptr := serr.JSONPointer // JSON pointer to unknown name
 //		name := ptr.LastToken() // unknown name itself
 //		...

src/encoding/json/v2/example_test.go

@@ -371,7 +371,8 @@ func Example_unknownMembers() {
 	// Specifying RejectUnknownMembers causes Unmarshal
 	// to reject the presence of any unknown members.
 	err = json.Unmarshal([]byte(input), new(Color), json.RejectUnknownMembers(true))
-	if serr, ok := errors.AsType[*json.SemanticError](err); ok && serr.Err == json.ErrUnknownName {
+	serr, ok := errors.AsType[*json.SemanticError](err)
+	if ok && serr.Err == json.ErrUnknownName {
 		fmt.Println("Unmarshal error:", serr.Err, strconv.Quote(serr.JSONPointer.LastToken()))
 	}