Update inkscape command in README.md

[gmlarumbe]

Hi,

When trying to render to PDF with the inkscape command suggested in the README.md I get the following error:

Unknown option --file

I am currently running version 1.3-2 of inkscape.

$ inkscape --version
Inkscape 1.3 (0e150ed6c4, 2023-07-21)

This PR updates the command in the README.md so that it works with latest versions of inkscape, using the following options instead:

inkscape -p --export-filename=mywave.pdf

Thanks!