corrected the serial number of 'source/installation/upgrade_319' and changed the heading size

Author: Tusharmalik

user_guide_src/source/installation/upgrade_319.rst

@@ -13,9 +13,9 @@ Replace all files and directories in your *system/* directory.
 .. note:: If you have any custom developed files in these directories,
 	please make copies of them first.
 
-********************************
-Step 3: Replace config/mimes.php
-********************************
+
+Step 2: Replace config/mimes.php
+================================
 
 This config file has received some updates. Please copy it to
 *application/config/mimes.php*.