Update documentation for HashPasswordChange

[LannyBose]

When referencing how to make a manual password change, I felt the included documentation could use some sprucing up.

Changes include:

• Ash.Changeset.new/2 seems to be gone
• Setting the context via set_context didn't work as expected when handed to HashPasswordChange.change. It kept resulting in BadMapError
• Providing explicit direction how to set an argument within the update Resource DSL

It's very possible my first chunk of code isn't idiomatic, but it did work. Happy to receive adjusting feedback there!