Fixed typo in header.

matlab/@dates/uminus.m

 function B = uminus(A)  % --*-- Unitary tests --*--
 
-% Overloads the unary plus operator for dates objects. Shifts all the elements by one period.
+% Overloads the unary minus operator for dates objects. Shifts all the elements by one period.
 %
 % INPUTS 
 %  o A    dates object with n elements.