[SparkR][Minor] Fix LDA doc

[junyangq]

What changes were proposed in this pull request?

This PR tries to fix the name of the SparkDataFrame used in the example. Also, it gives a reference url of an example data file so that users can play with.

How was this patch tested?

Manual test.

[SparkQA]

Test build #64548 has finished for PR 14853 at commit e179576.

• This patch fails R style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #64550 has finished for PR 14853 at commit 0a47e94.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[mengxr]

LGTM. Merged into master.