Workaround for Ruby::ArgumentTypeMismatch

```
lib/ruby_header_parser/data.rb:11:33: [error] Cannot pass a value of type `(::String | nil)` as an argument of type `::string`
│   (::String | nil) <: ::string
│     (::String | nil) <: (::String | ::_ToStr)
│       nil <: (::String | ::_ToStr)
│         nil <: ::String
│
│ Diagnostic ID: Ruby::ArgumentTypeMismatch
│
└       yaml = File.read(File.join(__dir__, "data.yml"))
                                   ~~~~~~~
```

Author: sue445

lib/ruby_header_parser/data.rb

@@ -8,7 +8,7 @@ class Data
     attr_reader :data
 
     def initialize
-      yaml = File.read(File.join(__dir__, "data.yml"))
+      yaml = File.read(File.join(__dir__.to_s, "data.yml"))
       @data = YAML.safe_load(yaml, aliases: true, permitted_classes: [Regexp])
     end