docs(readme): update svg logo to inherently support prefers-color-scheme

Author: jasonboukheir

Documentation~/images/algorand_logo_mark.svg

@@ -1,8 +1,6 @@
 <h1 id="unity-algorand-sdk" align="center">
 <picture>
-  <source media="(prefers-color-scheme: light)" srcset="Documentation~/images/algorand_logo_mark_black.svg">
-  <source media="(prefers-color-scheme: dark)" srcset="Documentation~/images/algorand_logo_mark_white.svg">
-  <img alt="The Algorand Logo" src="Documentation~/images/algorand_logo_mark_black.svg"/>
+  <img alt="The Algorand Logo" src="Documentation~/images/algorand_logo_mark.svg"/>
 </picture>
 <br/>
 Unity Algorand SDK