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>

Documentation/powerpc/syscall64-abi.rst

@@ -49,22 +49,22 @@ Register preservation rules
 Register preservation rules match the ELF ABI calling sequence with the
 following differences:
 
---- For the sc instruction, differences with the ELF ABI ---
-=========== ============= ========================================
-r0          Volatile      (System call number.)
-r3          Volatile      (Parameter 1, and return value.)
-r4-r8       Volatile      (Parameters 2-6.)
-cr0         Volatile      (cr0.SO is the return error condition.)
-cr1, cr5-7  Nonvolatile
-lr          Nonvolatile
-=========== ============= ========================================
-
---- For the scv 0 instruction, differences with the ELF ABI ---
-=========== ============= ========================================
-r0          Volatile      (System call number.)
-r3          Volatile      (Parameter 1, and return value.)
-r4-r8       Volatile      (Parameters 2-6.)
-=========== ============= ========================================
++------------------------------------------------------------------------+
+|        For the sc instruction, differences with the ELF ABI		 |
++--------------+--------------+------------------------------------------+
+| r0           | Volatile     | (System call number.)			 |
+| rr3          | Volatile     | (Parameter 1, and return value.)	 |
+| rr4-r8       | Volatile     | (Parameters 2-6.)			 |
+| rcr0         | Volatile     | (cr0.SO is the return error condition.)	 |
+| rcr1, cr5-7  | Nonvolatile  |						 |
+| rlr          | Nonvolatile  |						 |
++--------------+--------------+------------------------------------------+
+|      For the scv 0 instruction, differences with the ELF ABI		 |
++--------------+--------------+------------------------------------------+
+| r0           | Volatile     | (System call number.)			 |
+| r3           | Volatile     | (Parameter 1, and return value.)	 |
+| r4-r8        | Volatile     | (Parameters 2-6.)			 |
++--------------+--------------+------------------------------------------+
 
 All floating point and vector data registers as well as control and status
 registers are nonvolatile.