[README]: Updated License banner

MIT -> GNU

Author: MericLuc

README.md

@@ -1,4 +1,4 @@
-# path_finder ![Language](https://img.shields.io/badge/language-C++17-orange.svg) [![License](https://img.shields.io/badge/license-MIT-blue.svg)](./LICENSE.md)
+# path_finder ![Language](https://img.shields.io/badge/language-C++17-orange.svg) [![License](https://img.shields.io/badge/license-GNU-blue.svg)](./LICENSE.md)
 A simple templated binary search tree implementation
 **:star2: A-star Path-finding application :star2:**
 
@@ -144,4 +144,4 @@ It will install the following files :
 ${CMAKE_INSTALL_PREFIX}/path_finder
         path_finder
         default.json
-```
+```