gopls/internal/coverage: apply gofmt to the build tag syntax

Change-Id: I947cf23cdf1c7968cfacb9a97d374f06a51f9adb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/308491
Trust: Rebecca Stambler <rstambler@golang.org>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Peter Weinberger <pjw@google.com>
diff --git a/gopls/internal/coverage/coverage.go b/gopls/internal/coverage/coverage.go
index c3d30c6..7bb3640 100644
--- a/gopls/internal/coverage/coverage.go
+++ b/gopls/internal/coverage/coverage.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go.build go.1.16
+//go:build go.1.16
 // +build go.1.16
 
 // Running this program in the tools directory will produce a coverage file /tmp/cover.out