[MINOR][DOC] Remove spaces following slashs

## What changes were proposed in this pull request?

This PR merges multiple lines enumerating items in order to remove the redundant spaces following slashes in [Structured Streaming Programming Guide in 2.0.2-rc1](http://people.apache.org/~pwendell/spark-releases/spark-2.0.2-rc1-docs/structured-streaming-programming-guide.html).
- Before: `Scala/ Java/ Python`
- After: `Scala/Java/Python`
## How was this patch tested?

Manual by the followings because this is documentation update.

```
cd docs
SKIP_API=1 jekyll build
```

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #15686 from dongjoon-hyun/minor_doc_space.