Fix `byexample` testing readme.md

[DunklesArchipel]

byexample creates a cut-off at 80 characters and thus the output of the CLI is not complete in the readme.

Originally posted by @DunklesArchipel in https://github.com/echemdb/svgdigitizer/pull/182#discussion_r1154789704

[DunklesArchipel]

Displayed outputs can be modified by the +geometry option. The standard is +geometry 60x80 (output length width)

The following example returns short output for the CLI options.

$ svgdigitizer  # byexample: +geometry 60x40

But this does not allow for longer outputs

$ svgdigitizer  # byexample: +geometry 60x420

[DunklesArchipel]

It also seems that we can not suppress output warnings, which makes implementation of byexample even harder in #189