Update doc variable name in Lens exercises

[lukeknxt]

The guiding comments don't align with the actual code.

For example one sees

import monocle.macros.GenLens
val strNumber = GenLens[Address](_.strNumber)

val address = Address(10, "High Street")

And immediately after

streetNumber.get(address) should be(_)

streetNumber used instead of strNumber in the Address case class for consistency too.

This has been opened twice - #7 and #8, but they are either not approved or approved and forgotten about it seems.