debugcommands: fix a missing b prefix
This wasn't causing any immediate problems because of the source
transformer, but I noticed the inconsistency and it bugged me.
# skip-blame just a b prefix
Differential Revision: https://phab.mercurial-scm.org/D4255
<entry>
<title>{bookmark|escape}</title>
<link rel="alternate" href="{urlbase}{url|urlescape}rev/{node|short}"/>
<id>{urlbase}{url|urlescape}#bookmark-{node}</id>
<updated>{date|rfc3339date}</updated>
<published>{date|rfc3339date}</published>
<content type="text">{bookmark|strip|escape}</content>
</entry>