README: rename Associations section and fix mixed ivars
README: rename Associations section and fix mixed ivars

in code example (some were @q and others were @search).

[skip ci]

mariorcardoso/ransack,roman-franko/ransack,laserlemon/ransack,nagyt234/ransack,activerecord-hackery/ransack,Eric-Guo/ransack,AndrewSwerlick/ransack,vaski/ransack,amboxer21/ransack,guilleva/ransack,johan--/ransack,martndemus/ransack,Mario245/ransack,reinteractive/ransack,outstand/ransack,avit/ransack