Fixed missing import in combinatorics

Reran docs

book/src/list-functions-other.md

@@ -130,7 +130,7 @@ fn is_nonzero<D: Dim>(value: D) -> Bool
 </details>
 
 ### `is_int`
-Returns true if the input is an integer number of its units.
+Returns true if the input is an integer. If the input has units, then returns whether it is an integral number of its units.
 
 ```nbt
 fn is_int<D: Dim>(value: D) -> Bool

numbat/modules/math/combinatorics.nbt

@@ -1,5 +1,6 @@
 use core::error
 use core::functions
+use core::numbers
 use math::transcendental
 
 @name("Factorial")