[doc improve]How to import data from multiple tables in mysql into one tag/edge

[iiibui]

nebula-exchange version v2.6

As title, after some google I found this https://stackoverflow.com/questions/49130381/instead-of-fetching-multiple-tables-using-pyspark-how-can-we-execute-join-query, but here needs to be modified, and when I editing this issue I found this modification is same as:

nebula-exchange/nebula-exchange_spark_3.0/src/main/scala/com/vesoft/nebula/exchange/reader/ServerBaseReader.scala

Lines 77 to 82 in 92bbf5f

	if (sentence != null) {
	df.createOrReplaceTempView(mysqlConfig.table)
	session.sql(sentence)
	} else {
	df
	}

So, it would be better to add the usage to the documentation

[Nicole00]

Great issue, We'll support this requirement~