Make require name match the gem name

In order to require this gem you currently must use `"yard_sorbet"`

When you omit the yard prefix, yard automatically adds `yard-` to the
beginning of the plugin name. Because the file naming in this gem, that
means that the plugin will fail to load when running
`yard doc --plugin sorbet` and instead only works correctly by using the
underscored version of the gem name: `yard doc --plugin yard_sorbet`.

By changing the file names to use hyphens, `--plugin sorbet` as well as
`--plugin yard-sorbet` (i.e. the actual gem name) correctly work.

n.b. this will break any existing usages of `--plugin yard_sorbet`

Unfortunately, this is against the recommended gem naming guidelines at
https://guides.rubygems.org/name-your-gem/ but ultimately I think this
is the most user friendly option short of modifying yard to attempt both
`yard-` and `yard_` prefixes.

lib/yard_sorbet.rb → lib/yard-sorbet.rb

@@ -6,5 +6,5 @@
 # top-level namespace
 module YARDSorbet; end
 
-require_relative 'yard_sorbet/sig_handler'
-require_relative 'yard_sorbet/sig_to_yard'
+require_relative 'yard-sorbet/sig_handler'
+require_relative 'yard-sorbet/sig_to_yard'

spec/spec_helper.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require_relative '../lib/yard_sorbet'
+require_relative '../lib/yard-sorbet'
 
 if ENV['CI'] == 'true'
   require 'simplecov'

yard-sorbet.gemspec

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require_relative 'lib/yard_sorbet/version'
+require_relative 'lib/yard-sorbet/version'
 
 Gem::Specification.new do |spec|
   spec.name = 'yard-sorbet'