fix typo in session events docs
fix typo in session events docs

sandan/sqlalchemy,j5int/sqlalchemy,bdupharm/sqlalchemy,wujuguang/sqlalchemy,davidfraser/sqlalchemy,elelianghh/sqlalchemy,davidfraser/sqlalchemy,robin900/sqlalchemy,graingert/sqlalchemy,ThiefMaster/sqlalchemy,graingert/sqlalchemy,epa/sqlalchemy,ThiefMaster/sqlalchemy,epa/sqlalchemy,Akrog/sqlalchemy,j5int/sqlalchemy,hsum/sqlalchemy,itkovian/sqlalchemy,elelianghh/sqlalchemy,monetate/sqlalchemy,wujuguang/sqlalchemy,bdupharm/sqlalchemy,itkovian/sqlalchemy,robin900/sqlalchemy,sandan/sqlalchemy,zzzeek/sqlalchemy,hsum/sqlalchemy,brianv0/sqlalchemy,monetate/sqlalchemy,Akrog/sqlalchemy,sqlalchemy/sqlalchemy,brianv0/sqlalchemy