Update README.md

Author: ss892714028

README.md

@@ -6,7 +6,7 @@
 ### Symbols
 #### "@": Start and end node
 #### "#": Obstacles
-#### "<span style="color:green">*.*</span>": Visited node
+#### "<span style="color:green">***</span>": Visited node
 ##### "<span style="color:red">*+*</span>": Path