Bug in process associations - 'assoc.macro' renamed to 'assoc.methods'

[kornysietsma]

There seems to have been a bug introduced in this commit where assoc.macro.to_s was changed to assoc.methods.to_s in models_diagram.rb:

https://github.com/preston/railroady/blob/f348ba5a1694116032a97dc150d1b354282f597d/lib/railroady/models_diagram.rb#LL272C31-L272C31

This will break the processing of model associations.