arm64: alternatives: fix pr_fmt string for consistency

Consistently use the plural form for alternatives pr_fmt strings.
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/kernel/cpu_errata.c

@@ -16,7 +16,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#define pr_fmt(fmt) "alternative: " fmt
+#define pr_fmt(fmt) "alternatives: " fmt
 
 #include <linux/types.h>
 #include <asm/cpu.h>