Fix incomplete debian/watch rule

The upstream uses tags, not releases, so the newer version (2.2.5) is not detected correctly.

Before:

% uscan uscan warn: In debian/watch no matching files for watch line https://github.com/treasure-data/serverengine/releases ./v(?:[-_]?(\d[-+.:~\da-zA-Z]))(?i)(?:.(?:tar.xz|tar.bz2|tar.gz|tar.zstd?|zip|tgz|tbz|txz))

After:

uscan: Newest version of ruby-serverengine on remote site is 2.2.5, local version is 2.2.2 uscan: => Newer package available from: => https://github.com/treasure-data/serverengine/archive/refs/tags/v2.2.5.tar.gz Successfully repacked ../ruby-serverengine-2.2.5.tar.gz as ../ruby-serverengine_2.2.5.orig.tar.xz.