Fix setup instructions in README

[stinos]

After Install-Module the module is normally in the global module
directory so prefixing with .\ won't find it. Also strip the extension
since it is not needed and less strict.

[LinuxOnTheDesktop]

So: the README instructions need modifying.

[CodeByAidan]

2 years later, last time @DTW-DanWard (repo owner) has been on GitHub was May of 2022, 😞...
A more verbose way of doing this would be using the -Name parameter from PowerShell like

Import-Module -Name PowerShell-Beautifier