fix syntax highlighting in docs

Author: brean

docs/04_images.md

@@ -5,7 +5,7 @@ We use Pillow for this as it has a pretty straight-forward API.
 
 We define the green pixel in the image as start and the red pixel as end. To have some variation and combat compression we define them as ranges above a certain threshold (although the png-image in our lossless compressed):
 
-```
+```python
 def green(pixel: list) -> bool: 
     """Returns True if the pixel is green (the starting point)"""
     return pixel[RED] < 10 and pixel[GREEN] > 250 and pixel[BLUE] < 10
@@ -18,14 +18,14 @@ def red(pixel: list) -> bool:
 
 Pixel are defined as walkable grid cells if each of their color values is above 50, so a strong gray is enough to make it walkable:
 
-```
+```python
 def pixel_walkable(pixel, x, y):
     """returns True if the pixel is walkable."""
     return any([p > 50 for p in pixel])  # darker pixel are not walkable
 ```
 
 In the main loop we iterate over all pixel to find start and end and create a boolean matrix as input for our Grid:
-```
+```python
     with Image.open(filename_map) as im:
         width, height = im.size
         for y in range(height):