Fix broken markdown in docs (#306).

lib/src/rules/unnecessary_getters.dart

Index: lib/src/rules/unnecessary_getters.dart
diff --git a/lib/src/rules/unnecessary_getters.dart b/lib/src/rules/unnecessary_getters.dart
index 6cbd7ef09ce2da7d6099ebd321a252f515e8dff3..524bff01325bde9fda099ba95bae85893a7e45c3 100644
--- a/lib/src/rules/unnecessary_getters.dart
+++ b/lib/src/rules/unnecessary_getters.dart
@@ -13,7 +13,7 @@ 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.