update

docs/README.md

@@ -126,16 +126,16 @@ To control what API docs get built, you can set any combination of the following
 * `SKIP_RDOC=1`: Skip the R API docs.
 * `SKIP_SQLDOC=1`: Skip the SQL API docs.
 
-
 ## Build docs with docker image (Optional)
 
 As a Spark developer, you can generate all documents locally as follows:
 
+Note: Before running it, you need to have `docker` installed.
+
 ```sh
-sh dev/spark-test-image-util/docs/build-docs
+$ sh dev/spark-test-image-util/docs/build-docs
 ```
 
 It will generate all documents on the `container` and `host`.
 Especially when there are conflicts between the libraries required by Python development environment
 and the libraries required by generating Python docs environment, this is a good choice.
-Please refer to [SPARK-50295](https://issues.apache.org/jira/browse/SPARK-50295) for details.