Fix unclosed code block in README.md (prometheus#691)

This fixes a code block that not properly closed, causing the next two sections rendered as code.

Signed-off-by: Eric Yan <[email protected]>

Signed-off-by: Eric Yan <[email protected]>

Author: ericyan

README.md

@@ -197,6 +197,7 @@ docker run -d \
   --network my-mysql-network  \
   prom/mysqld-exporter
   --config.my-cnf=<path_to_cnf>
+```
 
 ## heartbeat