Added doctest for the NaiveDate years_since function (#1755)

Author: LucasBou

src/naive/date/mod.rs

@@ -1184,6 +1184,17 @@ impl NaiveDate {
     /// # Errors
     ///
     /// Returns `None` if `base > self`.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # use chrono::{NaiveDate};
+    /// #
+    /// let base: NaiveDate = "2025-01-01".parse().unwrap();
+    /// let date: NaiveDate = "2030-01-01".parse().unwrap();
+    ///
+    /// assert_eq!(date.years_since(base), Some(5))
+    /// ```
     #[must_use]
     pub const fn years_since(&self, base: Self) -> Option<u32> {
         let mut years = self.year() - base.year();