gopls/doc/release: fix (oddly) broken link

Change-Id: I6bd1f9950e7662eedd9f16bc46c12603858abf1c
Reviewed-on: https://go-review.googlesource.com/c/tools/+/691335
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>

Author: adonovan

gopls/doc/release/v0.20.0.md

@@ -15,7 +15,7 @@ web search index.
 ## Configuration changes
 
 This release enables by default the new persistent index of packages
-in the module cache. This was first attempted in [v0.19.0] but reverted
+in the module cache. This was first attempted in [v0.19](./v0.19.0.md) but reverted
 due to problems that have since been fixed.
 
 ## Web-based features