Fix the prose being inconsistent with the figure

[WhatisRT]

Description

Found by @HeinrichApfelmus. @TimSheard can you check that the second field in RewardUpdate does go into the reserves in the code?

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated. If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages. New section is never added with the code changes. (See RELEASING.md)
• Code is formatted with fourmolu (use scripts/fourmolize.sh)
• Cabal files are formatted (use scripts/cabal-format.sh)
• hie.yaml has been updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[HeinrichApfelmus]

Credit for prompting the cross-check between prose and figure goes to @KtorZ .

[KtorZ]

Was it me 😅? Maybe, I just remember complaining about stuff, which sounds a lot like me.

[TimSheard]

I carefully looked at the code, and explained what I found. So I believe the reserves are updated as they should be.