cmd/relnote: let 'path' be implicit, move 'title' up

No need to repeat the path when it's implicitly specified via file path
in x/website/_content. Also move the title key to be higher up. This is
to make the generator match what we've been doing in x/website the last
two releases.

For golang/go#64169.

Change-Id: Ic154bee804939e9e2a22bbe42d8aa108eba7e7d3
Reviewed-on: https://go-review.googlesource.com/c/build/+/633580
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: dmitshur

cmd/relnote/generate.go

@@ -15,9 +15,8 @@ import (
 )
 
 const prefixFormat = `---
-path: /doc/go1.%s
-template: false
 title: Go 1.%[1]s Release Notes
+template: false
 ---
 
 `