Fix example in atexit doc: Both open and read could raise the IOError (#10461 follow-up).

Thanks to SilenGhost for catching this.

Doc/library/atexit.rst

@@ -61,14 +61,11 @@ from a file when it is imported and save the counter's updated value
 automatically when the program terminates without relying on the application
 making an explicit call into this module at termination. ::
 
-   infile = open("/tmp/counter")
    try:
+       with open("/tmp/counter") as infile:
            _count = int(infile.read())
    except IOError:
        _count = 0
-   finally:
-       infile.close()
-
 
    def incrcounter(n):
        global _count