Incorrect asset extension file path in `AssetManager.h`

[Jack McPherson]

When running the engine, the startup process complains of a missing asset extension file. After hooking up GDB to the engine and single-stepping through the construction of GameEngine, a variable ASSET_EXTENSIONS_FILE is defined here like so:

limonEngine/src/Assets/AssetManager.h

Line 87 in 173cca9

 const std::string ASSET_EXTENSIONS_FILE = "./engine/assetExtensions.xml"; 

The actual directory is capitalised, i.e. Engine.

[Engin Manap]

merged.