docs: fix multiple doc build warnings in enumeration.rst

docs: fix multiple doc build warnings in enumeration.rst

The conversion of acpi/enumeration.txt to RST included one markup error,
leading to many warnings like:

  .../firmware-guide/acpi/enumeration.rst:430: WARNING: Unexpected indentation.

Add the missing colon and create some peace.

Fixes: 4c88b41c10e0 ("Documentation: ACPI: move enumeration.txt to firmware-guide/acpi and convert to reST")
Cc: Changbin Du <changbin.du@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>