gh-36071: Correct typo in class name of `Polynomial_ZZ_pEX` in `.pxd` file
<!-- ^^^^^
Corrects a typo in the cython definition of Polynomial_ZZ_pEX
-->
Currently the cpython definition of within
`/sage/rings/polynomial/polynomial_zz_pex.pxd` exports the class name
`Polynomial_ZZ_pX` instead of `Polynomial_ZZ_pEX`.
At the moment, if a user requires the `Polynomial_ZZ_pEX` within a
`.pyx` file, they must perform the following import:
```
from sage.rings.polynomial.polynomial_zz_pex cimport Polynomial_ZZ_pX as
Polynomial_ZZ_pEX
```
Correcting the typo in the changed file allows the import to be simply
```
from sage.rings.polynomial.polynomial_zz_pex cimport Polynomial_ZZ_pEX
```
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URL: https://github.com/sagemath/sage/pull/36071
Reported by: Giacomo Pope
Reviewer(s): Frédéric Chapoton
