correct deprecation messages

Author: dimpase

src/sage/rings/polynomial/multi_polynomial.pyx

@@ -21,7 +21,7 @@ from sage.misc.misc_c import prod
 
 def is_MPolynomial(x):
     from sage.misc.superseded import deprecation
-    deprecation(32709, "the function is_MPolynomial is deprecated; use isinstance(x, sage.structure.element.MPolynomial) instead")
+    deprecation(32709, "the function is_MPolynomial is deprecated; use isinstance(x, sage.rings.polynomial.multi_polynomial.MPolynomial) instead")
 
     return isinstance(x, MPolynomial)
 

src/sage/rings/polynomial/polynomial_element.pyx

@@ -176,7 +176,7 @@ cpdef is_Polynomial(f):
         False
     """
     from sage.misc.superseded import deprecation
-    deprecation(32709, "the function is_Polynomial is deprecated; use isinstance(x, sage.structure.element.Polynomial) instead")
+    deprecation(32709, "the function is_Polynomial is deprecated; use isinstance(x, sage.rings.polynomial.polynomial_element.Polynomial) instead")
 
     return isinstance(f, Polynomial)