Update README.md

README.md

@@ -176,7 +176,7 @@ Git advantage:
         graph BT
             classDef master fill:#418a44;
 
-            subgraph ref from HEAD
+            subgraph ref_from_HEAD
                 C1((C1));
                 C2((C2)) -- "HEAD~4" --> C1;
                 C3((C3)) -- "HEAD~^2~~" --> C1;