[VL][DOC] Update udf doc (apache#5814)

Update the compile command for build native udf library.

docs/developers/VeloxNativeUDF.md

@@ -137,7 +137,16 @@ You can also specify the local or HDFS URIs to the UDF libraries or archives. Lo
 ## Try the example
 
 We provided Velox UDF examples in file [MyUDF.cc](../../cpp/velox/udf/examples/MyUDF.cc) and UDAF examples in file [MyUDAF.cc](../../cpp/velox/udf/examples/MyUDAF.cc).
-After building gluten cpp, you can find the example libraries at /path/to/gluten/cpp/build/velox/udf/examples/
+You need to build the gluten cpp project with `--build_example=ON` to get the example libraries.
+
+```shell
+## compile Gluten cpp module
+cd /path/to/gluten/cpp
+## if you use custom velox_home, make sure specified here by --velox_home
+./compile.sh --build_velox_backend=ON --build_examples=ON
+```
+
+Then, you can find the example libraries at /path/to/gluten/cpp/build/velox/udf/examples/
 
 Start spark-shell or spark-sql with below configuration
 
@@ -157,16 +166,16 @@ or
 Run query. The functions `myudf1` and `myudf2` increment the input value by a constant of 5
 
 ```
-select myudf1(1), myudf2(100L)
+select myudf1(100L), myudf2(1)
 ```
 
 The output from spark-shell will be like
 
 ```
-+----------------+------------------+
-|udfexpression(1)|udfexpression(100)|
-+----------------+------------------+
-|               6|               105|
-+----------------+------------------+
++------------------+----------------+
+|udfexpression(100)|udfexpression(1)|
++------------------+----------------+
+|               105|               6|
++------------------+----------------+
 ```