gh-161078: Correct unstable C API PEP references

Author: TheLouisMahdi

Doc/whatsnew/3.12.rst

@@ -133,7 +133,7 @@ Security improvements:
 
 C API improvements:
 
-* :ref:`PEP 697 <whatsnew312-pep697>`, unstable C API tier
+* :ref:`PEP 689 <whatsnew312-pep689>`, unstable C API tier
 * :ref:`PEP 683 <whatsnew312-pep683>`, immortal objects
 
 CPython implementation improvements:
@@ -1861,9 +1861,9 @@ C API Changes
 New Features
 ------------
 
-.. _whatsnew312-pep697:
+.. _whatsnew312-pep689:
 
-* :pep:`697`: Introduce the :ref:`Unstable C API tier <unstable-c-api>`,
+* :pep:`689`: Introduce the :ref:`Unstable C API tier <unstable-c-api>`,
   intended for low-level tools like debuggers and JIT compilers.
   This API may change in each minor release of CPython without deprecation
   warnings.