Update URL for GN quick start guide.

Bug: None
Change-Id: I01ea2bb0508b018e219ecc94108e6a4fbe247984
Reviewed-on: https://boringssl-review.googlesource.com/31124
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

INCORPORATING.md

@@ -37,7 +37,7 @@ updating things more complex.
 
 BoringSSL is designed to work with many different build systems. Currently,
 different projects use [GYP](https://gyp.gsrc.io/),
-[GN](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/quick_start.md),
+[GN](https://gn.googlesource.com/gn/+/master/docs/quick_start.md),
 [Bazel](https://bazel.build/) and [Make](https://www.gnu.org/software/make/)  to
 build BoringSSL, without too much pain.