Fix doc on Days::new to refer to days, not months.

This fixes #871

src/naive/date.rs

@@ -126,7 +126,7 @@ impl NaiveWeek {
 pub struct Days(pub(crate) u64);
 
 impl Days {
-    /// Construct a new `Days` from a number of months
+    /// Construct a new `Days` from a number of days
     pub fn new(num: u64) -> Self {
         Self(num)
     }