[SPARKR][DOC] update doc for fpgrowth

[felixcheung]

What changes were proposed in this pull request?

minor update

@zero323

[SparkQA]

Test build #75503 has started for PR 17526 at commit e4e03ea.

[felixcheung]

Jenkins, retest this please

[SparkQA]

Test build #75505 has finished for PR 17526 at commit e4e03ea.

• This patch fails SparkR unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[felixcheung]

Jenkins, retest this please

[felixcheung]

First failure wasn't clear the cause, the second is related to streaming timing - I have seen it twice so far and should look into address it.

Failed -------------------------------------------------------------------------
1. Failure: read.stream, write.stream, awaitTermination, stopQuery (@test_streaming.R#56) 
head(sql("SELECT count(*) FROM people"))[[1]] not equal to 3.
1/1 mismatches
[1] 0 - 3 == -3


2. Failure: read.stream, write.stream, awaitTermination, stopQuery (@test_streaming.R#60) 
head(sql("SELECT count(*) FROM people"))[[1]] not equal to 6.
1/1 mismatches
[1] 3 - 6 == -3

[SparkQA]

Test build #75511 has finished for PR 17526 at commit e4e03ea.

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