Update simplediff.php

The following change fixes a small bug where if you have text on line 1 an empty line 2 and something else on line 3 and in the new text, you insert something into line 2, it will highlight the last word of line 1 and the first word of line 3 as being deletions. This should be merged into the Master branch I think

For example:

## Old text ##

This is line one.

This is line three.

## New text ##

This is line one.
Something is now here in line two
This is line three.

###
The only thing that should be highlighted is line 2 being an insert.