gh-140814: Fix call order in multiprocessing example

Move set_start_method() before freeze_support() in the "Safe importing
of main module" example.  freeze_support() internally accesses the start
method context, which causes a subsequent set_start_method() to raise
RuntimeError('context has already been set').

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: kovan

Doc/library/multiprocessing.rst

@@ -3238,8 +3238,8 @@ Safe importing of main module
            print('hello')
 
        if __name__ == '__main__':
-           freeze_support()
            set_start_method('spawn')
+           freeze_support()
            p = Process(target=foo)
            p.start()