Fix encoding error for IronPython compatibility

Replace non-ASCII characters (· and ×) with ASCII equivalents (dot and cross)
in comment on line 3931 to fix IronPython syntax error.

Co-authored-by: Licini <17893605+Licini@users.noreply.github.com>

Author: Copilot

src/compas/datastructures/mesh/mesh.py

@@ -3928,7 +3928,7 @@ def volume(self):
             # Each face is now a triangle (3 vertices)
             a, b, c = [vertices[i] for i in face]
             # Signed volume of tetrahedron formed by triangle and origin
-            # V = (1/6) * (a · (b × c)) where a, b, c are the vertices
+            # V = (1/6) * (a dot (b cross c)) where a, b, c are the vertices
             bc = cross_vectors(b, c)
             vol = dot_vectors(a, bc) / 6.0
             volume += vol