Fix typo in session docs
Fix typo in session docs

wujuguang/sqlalchemy,wujuguang/sqlalchemy,graingert/sqlalchemy,robin900/sqlalchemy,monetate/sqlalchemy,sqlalchemy/sqlalchemy,robin900/sqlalchemy,j5int/sqlalchemy,sandan/sqlalchemy,bdupharm/sqlalchemy,graingert/sqlalchemy,sandan/sqlalchemy,zzzeek/sqlalchemy,bdupharm/sqlalchemy,monetate/sqlalchemy,j5int/sqlalchemy