manual: correct the codeblock directive

patch by Kefu Chai; reviewed by Andy Tolbert and Bret McGuire
reference: #1940

Author: tchaikov

manual/mapper/daos/getentity/README.md

@@ -108,7 +108,7 @@ The method can return:
 * a single entity instance. If the argument is a result set type, the generated code will extract
   the first row and convert it, or return `null` if the result set is empty.
 
-    ````java
+    ```java
     @GetEntity
     Product asProduct(Row row);