fix typo

src/@dseries/isnan.m

 function b = isnan(o) % --*-- Unitary tests --*--
 
 % Returns an array of logicals (true/false). Element (t,i) is true iff the i-th variable at
-% period number t is not a NaN.
+% period number t is a NaN.
 %
 % INPUTS
 % - o [dseries]    with N variables and T periods.