Remove broken link from CHANGELOG. (#4131)

Author: davidmorgan

build/CHANGELOG.md

@@ -128,8 +128,7 @@ Internal changes:
 ## 2.1.0
 
 - Expand the expressiveness of `buildExtensions` include capture groups,
-  enabling directory moves. For more information on this feature, see
-  [capture groups](https://github.com/dart-lang/build/blob/master/docs/writing_a_builder.md#capture-groups)
+  enabling directory moves.
 - Add an `allowedOutputs` getter to `BuildStep`. It returns assets that may be
   written in that step.