DOC: update link to user guide
- [x] closes #39863 


Hello @morrme! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2021-02-17 19:18:12 UTC Built this locally and the link now works fine - thanks @morrme ! @jorisvandenbossche I put the 1.3 milestone, but now I'm seeing other doc changes (e.g. #39858) being backported - should I change to 1.2.3 and backport? This fixes a doc change concerning a 1.2 deprecation, so am pretty convinced it needs backporting

@MeeseeksDev backport to 1.2.x normally we don't backport too much except for CI issues that are likely to affect 1.2.x 
ok here if fixing something broken 