Remove a few double spaces

[AlekSi]

Description

And show how to get docs preview URL.

Readiness checklist

• [ ] I added/updated unit tests.
• [ ] I added/updated integration/compatibility tests.
• [ ] I added/updated comments and checked rendering.
• [ ] I made spot refactorings.
• [x] I updated user documentation.
• [x] I ran task all, and it passed.
• [x] I ensured that PR title is good enough for the changelog.
• [x] (for maintainers only) I set Reviewers (@FerretDB/core), Assignee, Labels, Project and project's Sprint fields.
• [x] I marked all done items in this checklist.

[codecov[bot]]

Codecov Report

Merging #2431 (e1149b2) into main (193c9d4) will increase coverage by 0.04%. The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2431      +/-   ##
==========================================
+ Coverage   64.28%   64.33%   +0.04%     
==========================================
  Files         392      392              
  Lines       19251    19251              
==========================================
+ Hits        12376    12385       +9     
+ Misses       5973     5963      -10     
- Partials      902      903       +1     

see 3 files with indirect coverage changes

Flag	Coverage Δ
integration	57.31% <ø> (ø)
mongodb	4.72% <ø> (ø)
pg	57.24% <ø> (ø)
unit	25.60% <ø> (+0.19%)	:arrow_up:

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