Update Design Doc: `level` tool

Design-Doc%3A-%60level%60-tool.md

@@ -44,7 +44,7 @@ The .lvl format is output by the `level` tool. It is a binary format ideal for u
 
 ### Entity Data
 ```
-2 byte: Number of entities
+2 bytes: Number of entities
 For (number of entities):
    * 2 bytes: Entity ID (-1 if not present)
 ```
@@ -68,4 +68,12 @@ For (number of component instances):
       * 2 bytes: Value of the associated field (order per configuration TOML as mentioned above).
 ```
 
+### Map Data
+```
+For plane A and plane B:
+   For (total tiles height):
+      For (total tiles width):
+         2 bytes: Tilemap ID
+```
+
 (TBD !!)