Replace the last rand() with mt_rand()

Better entropy, faster.
Also fixed a few "it's" typos.
diff --git a/system/database/DB_driver.php b/system/database/DB_driver.php
index 53decf0..618f366 100644
--- a/system/database/DB_driver.php
+++ b/system/database/DB_driver.php
@@ -1135,7 +1135,7 @@
 				else
 				{
 					/* We have no other choice but to just get the first element's key.
-					 * Due to array_shift() accepting it's argument by reference, if
+					 * Due to array_shift() accepting its argument by reference, if
 					 * E_STRICT is on, this would trigger a warning. So we'll have to
 					 * assign it first.
 					 */