fix links to heading IDs

[sheonhan]

In the Table of Contents, links to heading IDs had superfluous underscores, so the links weren't jumping to corresponding sections in the README.

[johnnyreilly]

Thanks for this! This arrived with #1032

The tricky thing is that the Table of Contents isn't hand cranked but generated using github-markdown-toc. If we fix it manually then presumably the issue will present again next time the TOC is regenerated.

Is there another way we could fix this "at source" as it were? cc @jsjoeio

[jsjoeio]

Thanks for catching this @sheonhan, and @johnnyreilly for tagging me.

Apologies - when I added it in, I didn't test every link. The majority were working so I thought it was good. I'll take a look to see what I can do to fix the underscores in the unique headings.

[sheonhan]

Thanks for such a quick response, everyone! I'll close this PR for now.