Fix bogus math expression in basic_formatter

[kfkonrad]

Description

One math expression had a simple syntax error.

Fixes #704

Environment report

Oh My Fish version:   6
OS type:              Darwin
Fish version:         fish, version 3.0.2
Git version:          git version 2.21.0 Git version:          hub version 2.11.2
Git core.autocrlf:    no
Your shell is ready to swim.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• New and existing tests pass locally with my changes

[derekstavis]

Seems like this was changed on fish 3.x. Merging as this is fish-spec specific.