docs: powerpc: syscall64-abi.rst: fix a malformed table

As reported:

	Documentation/powerpc/syscall64-abi.rst:53: WARNING: Malformed table.
	Text in column margin in table line 2.

	=========== ============= ========================================
	--- For the sc instruction, differences with the ELF ABI ---
	r0          Volatile      (System call number.)

This table requires a different notation to be valid.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>