Documentation: x86: fix boot.rst warning and format

Fix a Sphinx documentation format warning by breaking a long line
into 2 lines.

Also drop the ':' usage after the Protocol version numbers since
other Protocol versions don't use colons.

Documentation/x86/boot.rst:72: WARNING: Malformed table.
Text in column margin in table line 57.

Fixes: 2c33c27f ("x86/boot: Introduce kernel_info")
Fixes: 00cd1c15 ("x86/boot: Introduce kernel_info.setup_type_max")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/c6fbf592-0aca-69d9-e903-e869221a041a@infradead.org


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