[docs] Fixing broken link.

The link at the bottom of
https://gn.googlesource.com/gn/+/master/docs/standalone.md was 404'ing.
This patch fixes that link to point to the correct spot.

Change-Id: I549f4b6c487c0b6e800e3f5816d31fb78ca60c92
Reviewed-on: https://gn-review.googlesource.com/c/3260
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
diff --git a/docs/standalone.md b/docs/standalone.md
index 29d6236..08d589b 100644
--- a/docs/standalone.md
+++ b/docs/standalone.md
@@ -41,4 +41,4 @@
 
 If you want a completely standalone build that has nothing to do with Chrome
 and doesn't use Chrome's `//build` files, you can look at an example in
-[//tools/gn/example](../example).
+[//tools/gn/example](../tools/gn/example).