Deduplicate URL in Markdown

docs/r8_proguard.md

@@ -2,12 +2,11 @@ R8 / ProGuard
 =============
 
 If you use OkHttp as a dependency in an Android project which uses R8 as a default compiler you 
-don't have to do anything. The specific rules are [already bundled][rules] into the JAR which can be
+don't have to do anything. The specific rules are [already bundled][okhttp3_pro] into the JAR which can be
 interpreted by R8 automatically.
 
 If you, however, don't use R8 you have to apply the rules from [this file][okhttp3_pro]. You might
 also need rules from [Okio][okio] which is a dependency of this library.
 
  [okhttp3_pro]: https://github.com/square/okhttp/blob/master/okhttp/src/main/resources/META-INF/proguard/okhttp3.pro
  [okio]: https://square.github.io/okio/
- [rules]: https://github.com/square/okhttp/blob/master/okhttp/src/main/resources/META-INF/proguard/okhttp3.pro