fix(accordion): remove 'h5' from the card header
fix(accordion): remove 'h5' from the card header

Closes #2915 
fbasso/ng-bootstrap,fbasso/ng-bootstrap,fbasso/ng-bootstrap,ng-bootstrap/ng-bootstrap,benouat/ng-bootstrap,maxokorokov/core,ng-bootstrap/core,ng-bootstrap/ng-bootstrap,jnizet/ng-bootstrap,jnizet/ng-bootstrap,jnizet/ng-bootstrap,maxokorokov/core,ng-bootstrap/ng-bootstrap,benouat/ng-bootstrap,ng-bootstrap/core,maxokorokov/core,benouat/ng-bootstrap,maxokorokov/core,benouat/ng-bootstrap,ng-bootstrap/core,ng-bootstrap/ng-bootstrap,jnizet/ng-bootstrap,fbasso/ng-bootstrap