Update documentation of average.rs

[vil02]

Pull Request Template

Description

After merging #601, median allows f64 inputs. I have overlooked it back then.

Checklist:

• I ran bellow commands using the latest version of rust nightly.
• I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• I ran cargo fmt just before my last commit.
• I ran cargo test just before my last commit and all tests passed.
• I checked COUNTRIBUTING.md and my code follows its guidelines.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0f239fd) 94.40% compared to head (f21ee24) 94.40%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #609   +/-   ##
=======================================
  Coverage   94.40%   94.40%           
=======================================
  Files         268      268           
  Lines       21827    21827           
=======================================
  Hits        20605    20605           
  Misses       1222     1222           

Files	Coverage Δ
src/math/average.rs	100.00% <ø> (ø)

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