6382 - fix error on classDiagram.md

Fixing the error on the beginner's full action example in the class
diagram's documentation

Error: Error: Parse error on line 23:
...      callback Duck callback "Tooltip"
-----------------------^
Expecting 'NEWLINE', 'EOF', 'SQS', 'STR', 'DOT', 'GENERICTYPE', 'LABEL', 'STRUCT_START', 'STRUCT_STOP', 'STYLE_SEPARATOR', 'ANNOTATION_END', 'ALPHA', 'AGGREGATION', 'EXTENSION', 'COMPOSITION', 'DEPENDENCY', 'LOLLIPOP', 'LINE', 'DOTTED_LINE', 'CALLBACK_NAME', 'HREF', 'NUM', 'MINUS', 'UNICODE_TEXT', 'BQUOTE_STR', got 'CALLBACK'

packages/mermaid/src/docs/syntax/classDiagram.md

@@ -527,7 +527,7 @@ Beginner's tip—a full example using interactive links in an HTML page:
       +run()
       }
 
-      callback Duck callback "Tooltip"
+      callback Duck "callback" "Tooltip"
       link Zebra "https://www.github.com" "This is a link"
   </pre>