gopls/doc: add missing doc link

Change-Id: I691a03d5d48bee2a38a074d623c192cbb7ff3bd6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/626415
Auto-Submit: Alan Donovan <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: xieyuschen

gopls/doc/contributing.md

@@ -19,7 +19,7 @@ claiming it.
 ## Getting started
 
 Most of the `gopls` logic is in the `golang.org/x/tools/gopls/internal`
-directory. See [design/implementation.md] for an overview of the code organization.
+directory. See [design/implementation.md](./design/implementation.md) for an overview of the code organization.
 
 ## Build