update.sh: hide GITHUB_HEADERS missing output.

Will only happen once but could be confusing anyway.

Library/Homebrew/cmd/update.sh

@@ -477,7 +477,7 @@ EOS
         then
           # Only try to `git fetch` when the upstream tags have changed
           # (so the API does not return 304: unmodified).
-          GITHUB_API_ETAG="$(sed -n 's/^ETag: "\([a-f0-9]\{32\}\)".*/\1/p' ".git/GITHUB_HEADERS")"
+          GITHUB_API_ETAG="$(sed -n 's/^ETag: "\([a-f0-9]\{32\}\)".*/\1/p' ".git/GITHUB_HEADERS" 2>/dev/null)"
           GITHUB_API_ACCEPT="application/vnd.github.v3+json"
           GITHUB_API_ENDPOINT="tags"
         else