README: Update link to contributor guide

https://crrev.com/c/3557813 moved
/doc/book/src/contributing.md to /doc/book/scr/contributing/index.md

The link needs to be updated as well.

BUG=b:227345067
TEST=curl -I https://google.github.io/crosvm/contributing/
     shows HTTP/2 200

Change-Id: I6f659fde2d03da573f63cc656d95d24a253138b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3640903
Reviewed-by: Keiichi Watanabe <keiichiw@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chih-Yang Hsia <paulhsia@chromium.org>
Commit-Queue: Chih-Yang Hsia <paulhsia@chromium.org>

README.md

@@ -11,10 +11,9 @@ crosvm is currently used to run Linux/Android guests on Chrome OS devices.
 - [Documentation](https://google.github.io/crosvm/)
 - [Source code](https://chromium.googlesource.com/chromiumos/platform/crosvm/)
   - [API doc](https://google.github.io/crosvm/doc/crosvm/), useful for searching API.
-  - For contribution, see
-    [the contributor guide](https://google.github.io/crosvm/contributing.html). Mirror repository is
-    available at [GitHub](https://github.com/google/crosvm) for your convenience, but we don't
-    accept bug reports or pull requests there.
+  - For contribution, see [the contributor guide](https://google.github.io/crosvm/contributing/).
+    Mirror repository is available at [GitHub](https://github.com/google/crosvm) for your
+    convenience, but we don't accept bug reports or pull requests there.
 - [Issue tracker](https://bugs.chromium.org/p/chromium/issues/list?q=component:OS%3ESystems%3EContainers)
 
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