input: Documentation: corrections for event-codes.rst

Fix hyphenation, typos, capitalization, and a referenced file name
(.txt -> .rst).

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: linux-input@vger.kernel.org
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/20210302223523.20130-3-rdunlap@infradead.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>