remove mention of num_vertices and num_edges in the documentation

[etiennedeg]

fix #168

[codecov]

Codecov Report

Merging #169 (4484ca3) into master (b2a5b0b) will decrease coverage by 0.34%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #169      +/-   ##
==========================================
- Coverage   33.26%   32.92%   -0.35%     
==========================================
  Files           9        9              
  Lines         526      565      +39     
==========================================
+ Hits          175      186      +11     
- Misses        351      379      +28     

Impacted Files	Coverage Δ
src/GraphPlot.jl	100.00% <ø> (ø)
src/collapse_plot.jl	0.00% <0.00%> (ø)
src/deprecations.jl	0.00% <0.00%> (ø)
src/layout.jl	48.42% <0.00%> (+1.75%)	⬆️
src/lines.jl	52.22% <0.00%> (-0.12%)	⬇️
src/stress.jl	0.00% <0.00%> (ø)
src/pienode.jl	0.00% <0.00%> (ø)
src/plot.jl	62.63% <0.00%> (+0.56%)	⬆️
... and 1 more

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[etiennedeg]

I merge since this is only doc and should not be controversial.