Fix broken milestone links in README (#647)

Author: luispadron

README.md

@@ -31,9 +31,9 @@ don't have full support yet. See the
 for details on progress towards the 1.0 release. Here are a few of the planned
 high level features:
 
-- [ ] [Support all Core C/C++/Obj-C, rules_apple, and rules_swift rules](https://github.com/buildbuddy-io/rules_xcodeproj/milestone/1)
-- [ ] [Build Bazel targets with Bazel, in Xcode](https://github.com/buildbuddy-io/rules_xcodeproj/milestone/2)
-- [ ] [Easier target discovery and focused projects](https://github.com/buildbuddy-io/rules_xcodeproj/milestone/3)
+- [ ] [Support all Core C/C++/Obj-C, rules_apple, and rules_swift rules](https://github.com/buildbuddy-io/rules_xcodeproj/milestone/4)
+- [ ] [Build Bazel targets with Bazel, in Xcode](https://github.com/buildbuddy-io/rules_xcodeproj/milestone/8)
+- [ ] [Easier target discovery and focused projects](https://github.com/buildbuddy-io/rules_xcodeproj/milestone/12)
 
 We've also documented the [high level design](/doc/design/high-level.md) of the
 rules.