Fix doctests on Julia ≥ 1.7

[sostock]

StepRangeLen has an additional type parameter in newer Julia versions.

[codecov-commenter]

Codecov Report

Merging #29 (cbaf90c) into master (c76eb27) will increase coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #29      +/-   ##
==========================================
+ Coverage   95.43%   95.55%   +0.12%     
==========================================
  Files           6        6              
  Lines         548      518      -30     
==========================================
- Hits          523      495      -28     
+ Misses         25       23       -2     

Impacted Files	Coverage Δ
src/BSplines.jl	100.00% <ø> (ø)
src/spline.jl	97.72% <0.00%> (-0.10%)	⬇️
src/bsplinebasis.jl	100.00% <0.00%> (ø)
src/plotting.jl	34.37% <0.00%> (+2.02%)	⬆️

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