Update 'README.md'

README.md

@@ -5,6 +5,7 @@ A simple game which takes an image, then slices it into tiles and shuffles. You
 You only need a compiler supporting c++17 and sfml 2.5.1 (or latter versions).
 
 ### Usage
+```
 usage: sliding-puzzle [-OPTIONS...] FILE-OR-DIRECTORY
 
     Provide FILE-OR-DIRECTORY variable as a path to either a directory
@@ -21,3 +22,4 @@ usage: sliding-puzzle [-OPTIONS...] FILE-OR-DIRECTORY
                    by the smallest side of given source texture.
                    Hence, if your image is square, the amount of tiles
                    will be num * num.
+```