man: fix machine-id(5) man page reference

Spotted whilst debugging:
```
[763/2094] Generating man/machine-info.html with a custom command
Element cite in namespace '' encountered in para, but no template matches.
[765/2094] Generating man/machine-info.5 with a custom command
Element cite in namespace '' encountered in para, but no template matches.
```

Follow-up to 357376d0.