Fix default y axis title for jets input variables plots

Summary

The legend of a plot of h5 inputs variables displays something incorrect

Steps to reproduce

plot_input_variables.py -c ".../tests/zprime.yaml" --jets

What is the current bug behavior?

The y-axis of the plot of jets variables distribution (e.g IP3D_bu, pt_btagJes) displays "Normalised Number of Tracks"

What is the expected correct behavior?

It should display displays "Normalised Number of Jets"

Relevant logs and/or screenshots

Attached .png file is an example of such a plot with the wrong y-axis label

Possible fixes

https://gitlab.cern.ch/atlas-flavor-tagging-tools/algorithms/umami/-/blob/master/umami/input_vars_tools/PlottingFunctions.py#L72 The default value which is "Number of Tracks" should be replaced by "Number of Jets" when plotting jets, for instance here pt_btagJes