Fix wrong type in docs: i16 -> u16

@rustbot label +A-docs

r? docs

library/core/src/num/shells/u16.rs

@@ -1,4 +1,4 @@
-//! Redundant constants module for the [`i16` primitive type][i16].
+//! Redundant constants module for the [`u16` primitive type][u16].
 //!
 //! New code should use the associated constants directly on the primitive type.