tools: fix gopls update tool generation

Correctly select the non-prerelease version of gopls.

Change-Id: I229350bd4754614861c51f3c04f7d554fd3ed3c5
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/379934
Trust: Suzy Mueller <suzmue@golang.org>
Run-TryBot: Suzy Mueller <suzmue@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Trust: Nooras Saba‎ <saba@golang.org>
Reviewed-by: Nooras Saba‎ <saba@golang.org>
diff --git a/tools/generate.go b/tools/generate.go
index 3285157..41535a9 100644
--- a/tools/generate.go
+++ b/tools/generate.go
@@ -239,7 +239,8 @@
 	latestPre := versions.Versions[latestIndex]
 	// We need to find the last version that was not a pre-release.
 	var latest string
-	for latest = versions.Versions[latestIndex]; latestIndex >= 0; latestIndex-- {
+	for ; latestIndex >= 0; latestIndex-- {
+		latest = versions.Versions[latestIndex]
 		if !strings.Contains(latest, "pre") {
 			break
 		}