Fix link to iOS README in setup-building.rst (GH-1865)

Updated link to the iOS README for debugging test failures.

Author: raul-sq

getting-started/setup-building.rst

@@ -742,7 +742,7 @@ Platforms/Apple --help`` for more details.
 
 You can also run the test suite in Xcode itself. This is required if you want to
 run on a physical device. See the `iOS README
-<https://github.com/python/cpython/blob/main/iOS/README.rst#debugging-test-failures>`__
+<https://github.com/python/cpython/blob/main/Platforms/Apple/iOS/README.md#debugging-test-failures>`__
 for details.
 
 .. c_compile_and_build_end