powerpc: Fix typos and field name in comments

Fix a couple of typos and v_regs field name in mcontext_t.

	* sysdeps/unix/sysv/linux/powerpc/sys/ucontext.h: Fix typos and
	field name in mcontext_t struct.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>