Update README.md

README.md

@@ -237,7 +237,7 @@ Git advantage:
             graph BT
                 classDef dashed stroke-dasharray: 5, 5;
 
-                subgraph checkoutbranch
+                subgraph checkout_branch
                     C1a((C1));
                     C2a((C2)) --> C1a;
                     C3a((C3)) --> C2a;
@@ -255,7 +255,7 @@ Git advantage:
                     bc(branch) --> C2c;
                 end
 
-                subgraph move up
+                subgraph move_up
                     C1m((C1));
                     C2m(("->C2<-")) --> C1m;
                     bm(branch) --> C2m;