Fix a link in the README.

README.md

 `test` provides a standard way of writing and running tests in Dart.
 
 ## Writing Tests
 
 Tests are specified using the top-level [`test()`][test] function, and test
 assertions are made using [`expect()`][expect]:
 
 [test]: http://www.dartdocs.org/documentation/test/latest/index.html#test/test@id_test
 [expect]: http://www.dartdocs.org/documentation/test/latest/index.html#test/test@id_expect
 
 ```dart
 import "package:test/test.dart";
 
 void main() {
   test("String.split() splits the string on the delimiter", () {
     var string = "foo,bar,baz";
     expect(string.split(","), equals(["foo", "bar", "baz"]));
   });
 
   test("String.trim() removes surrounding whitespace", () {
     var string = "  foo ";
     expect(string.trim(), equals("foo"));
   });
 }
 ```
 
-Tests can be grouped together using the [`group()`] function. Each group's
-description is added to the beginning of its test's descriptions.
+Tests can be grouped together using the [`group()`][group] function. Each
+group's description is added to the beginning of its test's descriptions.
+
+[group]: http://www.dartdocs.org/documentation/test/latest/index.html#test/test@id_group
 
 ```dart
 import "package:test/test.dart";
 
 void main() {
   group("String", () {
     test(".split() splits the string on the delimiter", () {
       var string = "foo,bar,baz";
       expect(string.split(","), equals(["foo", "bar", "baz"]));
     });
@@ skipping 445 matching lines @@
 
 In this case, the port is `8081`. In another terminal, pass this port to
 `--pub-serve` and otherwise invoke `pub run test:test` as normal:
 
 ```shell
 $ pub run test:test --pub-serve=8081 -p chrome
 "pub serve" is compiling test/my_app_test.dart...
 "pub serve" is compiling test/utils_test.dart...
 00:00 +42: All tests passed!
 ```