update docs

docs/ex_004.md

@@ -63,7 +63,7 @@ However if you look at the Imageset, only the first symbol is present:
 
 ![tile_wall tiles](images/04-tile_wall-007.png)
 
-Non-text VERA display modes allow a vertical-flip and horizontal-flip flag to be set for each tile in a Tilemap, and the four tiles in our Aloevera Tilemap .png contain the unflipped original tile, the vertically-flipped version, the horizontally-flipped, and both flipped frames respectively. Aloevera is clever enough to recognise when an input Tile is the flipped version of a frame in the Imageset, and if it encouters a flipped tile it will output the original sorce index as well as the correct v and h flip fields at assembly time.
+Non-text VERA display modes allow a vertical-flip and horizontal-flip flag to be set for each tile in a Tilemap, and the four tiles in our Aloevera Tilemap .png contain the unflipped original tile, the vertically-flipped version, the horizontally-flipped, and both flipped frames respectively. Aloevera is clever enough to recognise when an input Tile is the flipped version of a frame in the Imageset, and if it encouters a flipped tile it will output the original index as well as the correct v and h flip fields at assembly time.
 
 ## More on Palette Alignment