readme: fixed screenshot images extension

Author: San7o

README.md

@@ -46,8 +46,8 @@ The engine also features the following sub projects:
 <h1 align=center> Screenshots </h1>
 
 <div align="center">
-  <img src="docs/screenshot1.png" />
-  <img src="docs/screenshot2.png" />
+  <img src="docs/screenshot1.jpg" />
+  <img src="docs/screenshot2.jpg" />
 </div>
 
 <h1 align=center> Building </h1>