PEP 790: fix reference to future version (#4496)

Author: JostMigenda

peps/pep-0790.rst

@@ -58,7 +58,7 @@ Subsequent bugfix releases every two months.
 
 * Python 3.15 will receive bugfix updates approximately every second month for
   two years.
-* Around the time of the release of 3.18.0 final, the final 3.15 bugfix update
+* Around the time of the release of 3.17.0 final, the final 3.15 bugfix update
   will be released.
 * After that, it is expected that security updates (source only) will be
   released for the next three years, until five years after the release of