Fix broken rustdoc intra-doc link in match_each_integer_ptype docs

The TODO note linked `match_each_unsigned_integer_ptype` without a
resolvable path, failing `RUSTDOCFLAGS=-D warnings cargo doc`. Qualify
it with the crate path.

Signed-off-by: Joe Isaacs <joe.isaacs@live.co.uk>

Author: joseph-isaacs

vortex-array/src/dtype/ptype.rs

@@ -498,7 +498,8 @@ macro_rules! match_each_native_ptype {
 ///
 /// This expands to 8 arms (all signed and unsigned integer widths). When the matched values are
 /// known to be non-negative (offsets, indices, lengths, run-lengths, counts), prefer
-/// [`match_each_unsigned_integer_ptype`] over a `PrimitiveArray::reinterpret_cast(ptype.to_unsigned())`
+/// [`match_each_unsigned_integer_ptype`](crate::match_each_unsigned_integer_ptype) over a
+/// `PrimitiveArray::reinterpret_cast(ptype.to_unsigned())`
 /// of the buffer: this halves each dimension (8 -> 4), and nested matches multiplicatively
 /// (8x8 -> 4x4), dramatically cutting monomorphized code size. The trick is only sound when the
 /// bound type is used to *read* values; if it also types an output buffer, build the output in the