stand: Fix a common typo in source code comments

- s/existance/existence/

MFC after:	3 days

stand/efi/include/eficon.h

@@ -421,7 +421,7 @@ EFI_STATUS
 
   Routine Description:
     Reads the next keystroke from the input device. The WaitForKey Event can
-    be used to test for existance of a keystroke via WaitForEvent () call.
+    be used to test for existence of a keystroke via WaitForEvent () call.
 
   Arguments:
     This       - Protocol instance pointer.

stand/i386/libi386/vbe.c

@@ -762,7 +762,7 @@ vbe_set_mode(int modenum)
 }
 
 /*
- * Verify existance of mode number or find mode by
+ * Verify existence of mode number or find mode by
  * dimensions. If depth is not given, walk values 32, 24, 16, 8.
  */
 static int

stand/libsa/geli/geliboot.c

@@ -165,7 +165,7 @@ geli_taste(geli_readfunc readfunc, void *readpriv, daddr_t lastsector,
 
 	/*
 	 * We have a new known_device.  Whether it's geli-encrypted or not,
-	 * record its existance so we can avoid doing IO to probe it next time.
+	 * record its existence so we can avoid doing IO to probe it next time.
 	 */
 	if ((kdev = malloc(sizeof(*kdev))) == NULL)
 		goto out;