minor fix for intro doc

[xxyzz]

I also wanna ask why plot3d's output feels weird when #:altitude is not 25:

(plot3d (polar3d (λ (θ ρ) 1) #:color 2 #:line-style 'transparent)
        #:altitude 0 #:angle 0)

Shouldn't this code output a circle instead of an ellipse?

[alex-hhh]

The plot package will include an #:aspect-ratio parameter which will help with controlling the aspect ratio of the plot area (although it does not work 100% correctly for 3D plots).

As for the actual documentation fix, I will close this pull request, as the change is too small and I want to discourage people from submitting low effort changes -- I have limited time as a maintainer and even such a small change takes a large chunk of my already tight time.

If you want to help out you can either:

• proofread and spell-check the plot manual (or a meaningful chunk of it)
• write the documentation for the new plot metrics feature (see Plotmetrics #90 and related issues)