docs: genericirq.rst: remove extra parenthesis in function definition

In the paragraph titled "Default flow implementations", the helper
function definition (simplified excerpt) for

noop(struct irq_data *data)

had an extraneous closing parenthesis. This commit removes the
unnecessary parenthesis, correcting the function definition.

Signed-off-by: Chih-Wei Chien <idoleat@taiker.tw>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240619160057.128208-1-idoleat@taiker.tw