Add missing mermaid markup (#5413)

## Which problem is this PR solving?
- Previous PR did not include markup and the diagram is not rendering
correctly

## Description of the changes
- Add markup

Signed-off-by: Yuri Shkuro <github@ysh.us>

cmd/jaeger/internal/extension/remotesampling/README.md

@@ -2,6 +2,7 @@
 
 Placeholder
 
+```mermaid
 flowchart LR
     Receiver --> AdaptiveSamplingProcessor --> BatchProcessor --> Exporter
     Exporter -->|"(1) get storage"| JaegerStorageExension
@@ -62,3 +63,4 @@ flowchart LR
             style SamplingStorage fill:blue,color:white
         end
     end
+```