fix: class of proxy method in AppiumClientConfig

[KazuCocoa]

Change list

Noticed the class of return value in proxy in AppiumClientConfig should be AppiumClientConfig

Types of changes

What types of changes are you proposing/introducing to Java client?
Put an x in the boxes that apply

• No changes in production code.
• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Details

ClientConfig should be AppiumClientConfig as same as other methods in the class