Fix prism_spot_def_for_name for singletons

[jhawthorn]

Previously calling a singleton method with invalid arguments would give:

Error: test_singleton_method_multiple_missing_keywords(ErrorHighlightTest): RuntimeError: Incompatible locations
/Users/jhawthorn/.rubies/ruby-dev/lib/ruby/4.0.0+0/prism/parse_result.rb:505:in 'Prism::Location#join'
/Users/jhawthorn/src/error_highlight/lib/error_highlight/base.rb:916:in 'ErrorHighlight::Spotter#prism_spot_def_for_name'
/Users/jhawthorn/src/error_highlight/lib/error_highlight/base.rb:295:in 'ErrorHighlight::Spotter#spot'
/Users/jhawthorn/src/error_highlight/lib/error_highlight/base.rb:68:in 'ErrorHighlight.spot'

This is because join wants the operator to come before the location