Fix a typographical error in the rustbook docs

[neunenak]

The modules anchor link was specified accidentally as #modules1190 instead of #modules.

[laniakea64]

Not sure that's an error?

On the website, that link is here: https://just.systems/man/en/listing-available-recipes.html#listing-available-recipes

and https://just.systems/man/en/modules.html is 404

[neunenak]

Ah my bad, this is what I get for trying to submit code at 3 am ._. the thing i was trying to fix was 1.6.38. Modules1.19.0 - so I changed the wrong string, but also it's werid that this html filename has the version string concatenated weirdly in it anyway. Will update this PR when I have a chance.

[neunenak]

Ok I don't know why this didn't work either. Honestly the Modules1.19.0 thing in the docs sidebar was just bugging me and though it would be easy to fix but I guess this involves debugging mdbook tooling.

[casey]

I think having version references in headings (and thus anchor links) is just generally insane, so I removed them in #3153.