tests: kernel: arm_irq_vector_table: minor typo and style fixes

Minor typo and style fixes in the test logging, stressing
that the test is applicable for Cortex-M MCUs, in general.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>
diff --git a/tests/kernel/arm_irq_vector_table/src/arm_irq_vector_table.c b/tests/kernel/arm_irq_vector_table/src/arm_irq_vector_table.c
index a31fadf..1f63411 100644
--- a/tests/kernel/arm_irq_vector_table/src/arm_irq_vector_table.c
+++ b/tests/kernel/arm_irq_vector_table/src/arm_irq_vector_table.c
@@ -101,7 +101,7 @@
  */
 void test_arm_irq_vector_table(void)
 {
-	printk("Test Cortex-M3 IRQ installed directly in vector table\n");
+	printk("Test Cortex-M IRQs installed directly in the vector table\n");
 
 	for (int ii = 0; ii < 3; ii++) {
 		irq_enable(_ISR_OFFSET + ii);