[improve docs]: Timestamp pallet (#1435)

This PR improves the docs for the Timestamp pallet by following our
[Documentation
Guidelines](https://github.com/paritytech/polkadot-sdk/blob/master/docs/DOCUMENTATION_GUIDELINE.md)
more closely.

---------

Co-authored-by: Juan <juangirini@gmail.com>
Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>

substrate/primitives/timestamp/src/lib.rs

@@ -140,7 +140,7 @@ pub enum InherentError {
 		error("The time since the last timestamp is lower than the minimum period.")
 	)]
 	TooEarly,
-	/// The block timestamp is too far in the future
+	/// The block timestamp is too far in the future.
 	#[cfg_attr(feature = "std", error("The timestamp of the block is too far in the future."))]
 	TooFarInFuture,
 }