Fix broken markdown in docs (#306).

lib/src/rules/unnecessary_getters.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library linter.src.rules.unnecessary_getters;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:linter/src/ast.dart';
 import 'package:linter/src/linter.dart';
 
 const desc =
     'Prefer using a public final field instead of a private field with a public getter.';
 
 const details = '''
-From the [style guide] (https://www.dartlang.org/articles/style-guide/):
+From the [style guide](https://www.dartlang.org/articles/style-guide/):
 
 **PREFER** using a public final field instead of a private field with a public
 getter.
 
 If you have a field that outside code should be able to see but not assign to
 (and you don't need to set it outside of the constructor), a simple solution
 that works in many cases is to just mark it `final`.
 
 **GOOD:**
 
@@ skipping 50 matching lines @@
     var candidates = getters.keys.where((id) => !setters.keys.contains(id));
     candidates.map((n) => getters[n]).forEach(_visitGetter);
   }
 
   _visitGetter(MethodDeclaration getter) {
     if (isSimpleGetter(getter)) {
       rule.reportLint(getter.name);
     }
   }
 }