gopls/internal/lsp/cache: fix use of time.NewTimer instead of NewTicker

Another facepalm. Discovered during manual testing of the prerelease.

Change-Id: I5fdb173a68ad8659d773259bcc737b4f708d23ab
Reviewed-on: https://go-review.googlesource.com/c/tools/+/512957
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

gopls/internal/lsp/cache/parse_cache.go

@@ -254,7 +254,7 @@ func (c *parseCache) startParse(mode parser.Mode, purgeFuncBodies bool, fhs ...s
 
 func (c *parseCache) gc() {
 	const period = 10 * time.Second // gc period
-	timer := time.NewTimer(period)
+	timer := time.NewTicker(period)
 	defer timer.Stop()
 
 	for {