doc: correct function reference in Timestamp

The documentation for the deprecated `as_duration` function in
`Timestamp` referenced a non-existent function `as_signed_duration`.
It should point to `as_jiff_duration` instead.

Closes #142

src/timestamp.rs

@@ -2132,7 +2132,7 @@ impl Timestamp {
     /// ```
     #[deprecated(
         since = "0.1.5",
-        note = "use Timestamp::as_signed_duration instead"
+        note = "use Timestamp::as_jiff_duration instead"
     )]
     #[inline]
     pub fn as_duration(self) -> (i8, core::time::Duration) {