Update sample.md so that mermaid diagrams could be displayed properly (…

…#1264)

In the original `sample.md` file, a <div> with class sets to mermaid is used to enclose the mermaid code. However, this does not properly display the mermaid diagram. After changing it to ```mermaid ```, the expected behavior could be observed.

sample.md

@@ -66,7 +66,7 @@ Merged       | Cell 1
 
 ---
 
-<div class="mermaid">
+```mermaid
 sequenceDiagram
     participant Alice
     participant Bob 
@@ -78,19 +78,19 @@ sequenceDiagram
     John-->>Alice: Great!
     John->>Bob: How about you?
     Bob-->>John: Jolly good!
-</div> 
+```
 
 ---
 
-<div class="mermaid">
+```mermaid
 graph LR;
   A(AAAA)==> B(B node);
   B==> C(SEE SEE);
 
   class A diag-a-styles;
   class B diag-b-styles
   class C diag-c-styles;
-</div>
+```
 
 ---