docs: refine README title hierarchy

Author: ReverseZoom2151

README.md

@@ -1,8 +1,10 @@
 <h1 align="center">
   <img src="doom-guy.gif" width="48" alt="DOOM Guy">
-  GameNGen: Neural Game Engine Research Implementation
+  GameNGen
 </h1>
 
+<p align="center"><strong>Neural Game Engine Research Implementation</strong></p>
+
 An in-progress, test-driven research implementation inspired by the ICLR 2025
 paper [*Diffusion Models Are Real-Time Game Engines*](https://arxiv.org/abs/2408.14837).