Fixup broken links

```
dmr on titan in synapse/book on  dmr/docs-tidy via  v16.14.0 via 🐍 v3.10.7 (matrix-synapse-py3.10) via 🐏 12GiB/15GiB | 5GiB/8GiB took 8s
2022-10-24 15:15:53 ✔  $ linkchecker **.html
INFO linkcheck.cmdline 2022-10-24 15:16:33,785 MainThread Checking intern URLs only; use --check-extern to check extern URLs.
LinkChecker 10.1.0
Copyright (C) 2000-2016 Bastian Kleineidam, 2010-2021 LinkChecker Authors
LinkChecker comes with ABSOLUTELY NO WARRANTY!
This is free software, and you are welcome to redistribute it under
certain conditions. Look at the file `LICENSE' within this distribution.
Read the documentation at https://linkchecker.github.io/linkchecker/
Write comments and bugs to https://github.com/linkchecker/linkchecker/issues

Start checking at 2022-10-24 15:16:33+001
/usr/lib/python3.10/site-packages/bs4/__init__.py:435: MarkupResemblesLocatorWarning: The input looks more like a filename than markup. You may want to open this file and pass the filehandle into Beautiful Soup.
  warnings.warn(
10 threads active,    88 links queued,  241 links in 339 URLs checked, runtime 1 seconds

Statistics:
Downloaded: 4.04MB.
Content types: 7 image, 106 text, 0 video, 0 audio, 22 application, 3 mail and 508 other.
URL lengths: min=16, max=256, avg=67.

That's it. 646 links in 646 URLs checked. 0 warnings found. 0 errors found.
Stopped checking at 2022-10-24 15:16:39+001 (5 seconds)
```

0 files changed, 0 insertions, 0 deletions