Fix broken markdown in docs (#306).

lib/src/rules/slash_for_doc_comments.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.slash_for_doc_comments;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:linter/src/linter.dart';
 
 const desc = r'Prefer to use /// for doc comments';
 
 const details = r'''
-From the [style guide] (https://www.dartlang.org/articles/style-guide/):
+From the [style guide](https://www.dartlang.org/articles/style-guide/):
 
 **PREFER** to use `///` for doc comments.
 
 Although Dart supports two syntaxes of doc comments (`///` and `/**`), we 
 prefer using `///` for doc comments.
 
 **GOOD:**
 ```
 /// Parses a set of option strings. For each option:
 ///
@@ skipping 86 matching lines @@
   @override
   visitMethodDeclaration(MethodDeclaration node) {
     checkComment(node.documentationComment);
   }
 
   @override
   visitTopLevelVariableDeclaration(TopLevelVariableDeclaration node) {
     checkComment(node.documentationComment);
   }
 }