#18601: fix error made when difflib example was converted to use 'with'.

Doc/library/difflib.rst

@@ -752,7 +752,7 @@ It is also contained in the Python source distribution, as
        # we're passing these as arguments to the diff function
        fromdate = time.ctime(os.stat(fromfile).st_mtime)
        todate = time.ctime(os.stat(tofile).st_mtime)
-       with open(fromlines) as fromf, open(tofile) as tof:
+       with open(fromfile) as fromf, open(tofile) as tof:
            fromlines, tolines = list(fromf), list(tof)
 
        if options.u: