docs: fault-injection: fix non-working usage of negative values

docs: fault-injection: fix non-working usage of negative values

Fault injection uses debugfs in a way that the provided values via sysfs
are interpreted as u64. Providing negative numbers results in an error:

/sys/kernel/debug/fail_function# echo -1 > times
sh: write error: Invalid argument

Update the docs and examples to use "printf %#x <val>" in these cases.
For "retval", reword the paragraph a little and fix a typo.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20210603125841.27436-1-wsa+renesas@sang-engineering.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>