How can I calculate inductance in this page?
No - that is not the relative permeability.

They already calculated the effect of the permeability and the magnetic path of the core into a more useful number: \$A_L\$.  This is the inductance in nano-henries that a given number of wire turns on the core will give. The inductance goes up as the square of the number of turns because of mutual inductance between the turns.

For example if you put 10 turns on the core, this will give 10² * 50 = 5000nH.

A description of \$A_L\$ is here.