Improve sentence

Co-authored-by: Vincent Macri <vincent.macri@ucalgary.ca>

src/sage/rings/function_field/jacobian_hess.py

@@ -606,7 +606,7 @@ def _element_constructor_(self, x):
         """
         Construct an element of ``self`` from ``x``.
 
-        If ``x`` is an effective divisor, then it is assumed to be of
+        If ``x`` is an effective divisor, then it must be of
         degree `g`, the genus of the function field.
 
         EXAMPLES::