Fix install instructions

[robzolkos]

The installer is a generator not a rake task (from my testing)

Refer https://github.com/rails/solid_cable/blob/main/lib/generators/solid_cable/install/USAGE#L5

[npezza93]

We actually have both. The rake task just calls the generator.

https://github.com/rails/solid_cable/blob/main/lib/tasks/solid_cable_tasks.rake

[robzolkos]

I got this when following the README:

$ bin/rails solid_cable:install
Unrecognized command "solid_cable:install" (Rails::Command::UnrecognizedCommandError)
Did you mean?  solid_cable:install:migrations

[robzolkos]

Never mind, I see I have 1.0.3 not latest.