Fix documentation bug on how to enable batch mode for streaming examples

In latest flink doc, it says that we should use '-Dexecution.runtime-mode=BATCH' to enable batch mode，but it does not work actually. The wrong way is as below:

bin/flink run -Dexecution.runtime-mode=BATCH examples/streaming/WordCount.jar

we should use '--execution-mode batch' instead, the correct way is as below

bin/flink run examples/streaming/WordCount.jar --execution-mode batch