[Doc] Add vec5f definition in example.

docs/lang/articles/math/math_module.md

@@ -113,6 +113,7 @@ u = v.rraa  # vec4(1, 1, 2, 2)
 - `ti.Vector` is a function that accepts a 1D array and returns a matrix instance that has only one column. For example, `ti.Vector([1, 2, 3, 4, 5])`.
 - `ti.types.vector` is a function that accepts an integer and a primitive type and returns a vector type. For example: `vec5f = ti.types.vector(5, float)`. `vec5f` can then be used to instantiate 5D vectors or annotate data types of function arguments and struct members:
     ```python
+    vec5f = ti.types.vector(5, float)
     @ti.kernel
     def test(v: vec5f):
         print(v.xyz)