gh-38286: `is_NumberFieldElement`: Fix deprecation message
    
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The current message is referring to a class that does not exist.

- Part of #32414

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### ⌛ Dependencies

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URL: #38286
Reported by: Matthias Köppe
Reviewer(s): Travis Scrimshaw

Author: Release Manager

src/sage/rings/number_field/number_field_element.pyx

@@ -104,7 +104,7 @@ def is_NumberFieldElement(x):
         sage: is_NumberFieldElement(2)
         doctest:warning...
         DeprecationWarning: is_NumberFieldElement is deprecated;
-        use isinstance(..., sage.structure.element.NumberFieldElement) instead
+        use isinstance(..., sage.rings.number_field.number_field_element_base.NumberFieldElement_base) instead
         See https://github.com/sagemath/sage/issues/34931 for details.
         False
         sage: x = polygen(ZZ, 'x')
@@ -115,7 +115,7 @@ def is_NumberFieldElement(x):
     from sage.misc.superseded import deprecation
     deprecation(34931,
                 'is_NumberFieldElement is deprecated; '
-                'use isinstance(..., sage.structure.element.NumberFieldElement) instead')
+                'use isinstance(..., sage.rings.number_field.number_field_element_base.NumberFieldElement_base) instead')
     return isinstance(x, NumberFieldElement)