Fix L1 error misnomer in the Minterpy regression

In the Minterpy ordinary regression, once a polynomial regression has been fitted, some measures of fitting errors may be queried as properties, i.e., L-2 (RMSE) and L-\infty errors. However, there is a mistake in the code that designates the L-\infty error as L-1, i.e., the regfit_l1_error property.

This mistake should be fixed by renaming the property.