x86/boot/Documentation: Fix htmldocs build warning due to malformed table in boot.rst

Sphinx reports htmldocs warnings:

  Documentation/arch/x86/boot.rst:437: ERROR: Malformed table.
  Text in column margin in table line 2.

The table header defined the first column width as 2 characters ("=="),
which is too narrow for entries like "0x10" and "0x13". This caused the
text to spill into the margin, triggering a docutils parsing failure.

Fix it by extending the first column of assigned boot loader ID to 4
characters ("====") to fit the widest entries.

Fixes: 1c3377be ("x86/boot/Documentation: Prefix hexadecimal literals with 0x")
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Swaraj Gaikwad <swarajgaikwad1925@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://patch.msgid.link/20251210092814.9986-1-swarajgaikwad1925@gmail.com