Fix broken link in docs

The target file was moved in prompt-toolkit
on the commit
https://github.com/prompt-toolkit/python-prompt-toolkit/commit/837609dd55162137ff7a177a8e293cb7a94f3980

docs/source/config/details.rst

@@ -247,7 +247,7 b' VI input mode to ``Normal`` when in insert mode::'
 
 For more information on filters and what you can do with the ``event`` object,
 `see the prompt_toolkit docs
-<http://python-prompt-toolkit.readthedocs.io/en/latest/pages/building_prompts.html#adding-custom-key-bindings>`__.
+<https://python-prompt-toolkit.readthedocs.io/en/latest/pages/asking_for_input.html#adding-custom-key-bindings>`__.
 
 
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