Update README.md

README.md

@@ -237,7 +237,7 @@ Git advantage:
             graph BT
                 classDef dashed stroke-dasharray: 5, 5;
 
-                subgraph checkout branch
+                subgraph checkoutbranch
                     C1a((C1));
                     C2a((C2)) --> C1a;
                     C3a((C3)) --> C2a;