[MINOR][DOC] Correct code snippet results in quick start documentation

## What changes were proposed in this pull request?

As README.md file is updated over time. Some code snippet outputs are not correct based on new README.md file. For example:
```
scala> textFile.count()
res0: Long = 126
```
should be
```
scala> textFile.count()
res0: Long = 99
```
This pr is to add comments to point out this problem so that new spark learners have a correct reference.
Also, fixed a samll bug, inside current documentation, the outputs of linesWithSpark.count() without and with cache are different (one is 15 and the other is 19)
```
scala> val linesWithSpark = textFile.filter(line => line.contains("Spark"))
linesWithSpark: org.apache.spark.rdd.RDD[String] = MapPartitionsRDD[2] at filter at <console>:27

scala> textFile.filter(line => line.contains("Spark")).count() // How many lines contain "Spark"?
res3: Long = 15

...

scala> linesWithSpark.cache()
res7: linesWithSpark.type = MapPartitionsRDD[2] at filter at <console>:27

scala> linesWithSpark.count()
res8: Long = 19
```

## How was this patch tested?

manual test:  run `$ SKIP_API=1 jekyll serve --watch`

Author: linbojin <linbojin203@gmail.com>

Closes #14645 from linbojin/quick-start-documentation.