arm64: fix spelling mistake "ca not" -> "cannot"

There is a spelling mistake in the comment, Fix it.
Signed-off-by: hankecai <hankecai@bbktel.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/lib/strcmp.S

@@ -186,7 +186,7 @@ CPU_LE( rev	data2, data2 )
 	* as carry-propagation can corrupt the upper bits if the trailing
 	* bytes in the string contain 0x01.
 	* However, if there is no NUL byte in the dword, we can generate
-	* the result directly.  We ca not just subtract the bytes as the
+	* the result directly.  We cannot just subtract the bytes as the
 	* MSB might be significant.
 	*/
 CPU_BE( cbnz	has_nul, 1f )