Update filename example in initial Steepfile

[tk0miya]

As far as I know, Steep does not support type checking Gemfile. Therefore it's not good to use Gemfile as an example for specifying specific file.

This replaces it with actual .rb file case.

As a trial, I uncommented the line and tried to check Gemfile. But I got the following errors:

tkomiya@tarf> bundle exec steep check                                                                                                                                      ~/work/tmp/tmptmp
# Type checking files:

...................................................................F................

Gemfile:3:0: [error] Type `::Object` does not have method `source`
│ Diagnostic ID: Ruby::NoMethod
│
└ source "https://rubygems.org"
  ~~~~~~

Gemfile:5:0: [error] Type `::Object` does not have method `gem`
│ Diagnostic ID: Ruby::NoMethod
│
└ gem "steep", require: false
  ~~~

Detected 2 problems from 1 file