Fix annotation in CodeObjects::Base#files

The original annotation incorrectly was Array<String> but the format is

```
[ [file, line] ]
```

Therefore the annotation should be `Array<Array<String, Integer>>`,
given there is no prescribed way to describe tuples

lib/yard/code_objects/base.rb

@@ -132,7 +132,7 @@ def push(value)
     # @see NamespaceMapper.register_separator
     class Base
       # The files the object was defined in. To add a file, use {#add_file}.
-      # @return [Array<String>] a list of files
+      # @return [Array<Array<String, Integer>>] a list of files
       # @see #add_file
       attr_reader :files