Fixed link to "Challenges of Writing Rules" from the contributing page.

--
Change-Id: I759cf1c3b102a9fb4a755f817f7c420c8cd01bb7
Reviewed-on: https://bazel-review.googlesource.com/c/5690/
MOS_MIGRATED_REVID=131833178
diff --git a/site/contributing.md b/site/contributing.md
index e519115..a815313 100644
--- a/site/contributing.md
+++ b/site/contributing.md
@@ -142,7 +142,7 @@
     first.
   * Builtin rules in `com.google.devtools.build.lib.rules` and in
     `com.google.devtools.build.lib.bazel.rules`. You might want to read about
-    the [Challenges of Writing Rules](rule-challenges.html) first.
+    the [Challenges of Writing Rules](docs/rule-challenges.html) first.
 * Java native interfaces in `src/main/native`.
 * Various tooling for language support (see the list in the
   [compiling Bazel](#compile-bazel) section).