docs: correct Figure 6 revision label

[nightcityblade]

Fixes #6060

This updates the Figure 6 diagram in the consistency docs so the lower-left write path is labeled Save with _rev1, matching the conflict scenario described in the issue.

Verification:

• visually checked the rendered PNG after the edit

[rnewson]

hm the '1' there isn't readable, it's halfway under the adjacent image.

We seem to have lost the original source files for these diagrams so perhaps it's time for an overhaul.

At @kxepal's suggestion I looked at mermaid and think we should switch all these diagrams over to that. The editable source can be checked in to the src/docs subdir and a makefile target can then convert them to PNG and/or SVG at build time.

As an example, I did the first diagram;

sequenceDiagram
    Songbird->>CouchDB: Initial backup
    CouchDB-->>Songbird: Reply with rev 1
    CouchDB->>Laptop: Restore from backup on new laptop
    Laptop->>Laptop: User edits playlist
    Laptop->>CouchDB: Send update with rev 1
    CouchDB-->>Laptop: Reply with rev 2
    Songbird->>CouchDB: Send update with rev 1
    CouchDB-->>Songbird: 409 Conflict

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Is this something you'd be interested in working on?

[nightcityblade]

Thanks for the context and the mermaid example. I’m interested in that broader conversion, but it’s a different scope than this one-line label fix, so I don’t want to quietly expand this PR into a docs overhaul. I’ll leave this PR as-is and can open a separate follow-up for the mermaid migration work instead if that would be helpful.