docs(flowchart): fix the example code of Markdown Strings

docs/syntax/flowchart.md

@@ -1600,6 +1600,7 @@ flowchart LR
 The "Markdown Strings" feature enhances flowcharts and mind maps by offering a more versatile string type, which supports text formatting options such as bold and italics, and automatically wraps text within labels.
 
 ```mermaid-example
+---
 config:
   flowchart:
     htmlLabels: false
@@ -1616,6 +1617,7 @@ end
 ```
 
 ```mermaid
+---
 config:
   flowchart:
     htmlLabels: false