Documentation: fix core_pattern max length

The buffer size for core_pattern is 128, but one character is used for
terminating null byte, so the actual limit is 127:

    # printf '%0999d' > /proc/sys/kernel/core_pattern
    # tr -d '\n' < /proc/sys/kernel/core_pattern | wc -c
    127

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