Fix the link to Skyframe in the contributing page

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Change-Id: I33742e39d6620eed9c059cb6c008a9839ae35149
Reviewed-on: https://bazel-review.googlesource.com/5730
MOS_MIGRATED_REVID=131845872

site/contributing.md

diff --git a/site/contributing.md b/site/contributing.md
index a815313..f5e72c5 100644
--- a/site/contributing.md
+++ b/site/contributing.md
@@ -134,7 +134,7 @@
 * Client code in `src/main/cpp` provides the command-line interface.
 * Protocol buffers in `src/main/protobuf`.
 * Server code in `src/main/java` and `src/test/java`.
-  * Core code which is mostly composed of [SkyFrame](docs/skyframe.html) and some
+  * Core code which is mostly composed of [SkyFrame](designs/skyframe.html) and some
     utilities.
   * Rules written in Bazel's extension language
     [Skylark](docs/skylark/index.html) are defined in `tools/build_rules`. If