[docs] [issue36822] Minor grammatical fix in glossary.rst

[SilentGhost]

Change by SilentGhost <ghost.adh at runbox.com>:


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type: enhancement -> behavior
versions:  -Python 3.5, Python 3.6, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue36822>
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