Documentation: kgdb: properly capitalize the MAGIC_SYSRQ config

Most readers are probably going to figure out that the config is actually
all upper-case letters, as all Kconfig symbols are this way.

Properly capitalizing makes the script ./scripts/checkkconfigsymbols.py
happy, which otherwise would report this as a reference to a non-existing
Kconfig symbol.

So, use the right capitalization for the MAGIC_SYSRQ config in the kgdb
documentation.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20211230172423.30430-1-lukas.bulwahn@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>