Fix: Add JSDoc to fix linter error

[nikolasrieble]

This PR by @saghan broke the CI pipeline, which now fails due to a linter error.

Run tsc --noEmit

> recharts@2.1.13 lint /home/runner/work/recharts/recharts
> eslint './src/**/*.?(ts|tsx)'


/home/runner/work/recharts/recharts/src/cartesian/CartesianAxis.tsx
Error:   450:3  error  Missing JSDoc for parameter 'fontSize'       valid-jsdoc
Error:   450:3  error  Missing JSDoc for parameter 'letterSpacing'  valid-jsdoc

Adding JSDoc for two arguments is sufficient to fix this.

[saghan]

Thank you, the fix is also there in this #2949

[nikolasrieble]

I will leave this open in case the other PR does not get merged. Once the other one is merged, this can be closed either way.