bpo-31537: Update readline documentation example. (GH-3925)

Change the code example from using `get_history_length` to `get_current_history_length`.

Doc/library/readline.rst

@@ -312,13 +312,13 @@ sessions, by only appending the new history. ::
 
    try:
        readline.read_history_file(histfile)
-       h_len = readline.get_history_length()
+       h_len = readline.get_current_history_length()
    except FileNotFoundError:
        open(histfile, 'wb').close()
        h_len = 0
 
    def save(prev_h_len, histfile):
-       new_h_len = readline.get_history_length()
+       new_h_len = readline.get_current_history_length()
        readline.set_history_length(1000)
        readline.append_history_file(new_h_len - prev_h_len, histfile)
    atexit.register(save, h_len, histfile)

Misc/NEWS.d/next/Documentation/2017-10-08-23-02-14.bpo-31537.SiFNM8.rst

@@ -0,0 +1,2 @@
+Fix incorrect usage of ``get_history_length`` in readline documentation
+example code. Patch by Brad Smith.