TheStreet's Action Alerts PLUS co-portfolio manager Jim Cramer says the selloff in Boeing (BA) shares on Wednesday is overdone. Cramer says investors who own shares should stay the course, and for others the next few days could be a buying opportunity. Cramer says Boeing is a great multi-year story and the Farnborough International Airshow in July should give investors another look at orders that could be placed by airlines. Cramer says the only reason for the Boeing downgrade was that the analyst said shares had run-up too high. Cramer says there is no single data point that signals Boeing is finished.