Doc fix: The Imputer is an Estimator

[zoltanctoth]

Fixing the doc as the imputer is not a Transformer but an Estimator.

https://github.com/apache/spark/blob/master/mllib/src/main/scala/org/apache/spark/ml/feature/Imputer.scala#L96-L97

What changes were proposed in this pull request?

Simple documentation fix

How was this patch tested?

manual testing

Please review http://spark.apache.org/contributing.html before opening a pull request.

[SparkQA]

Test build #4214 has finished for PR 21755 at commit 19cd33b.

• This patch fails Spark unit tests.
• This patch does not merge cleanly.
• This patch adds no public classes.

[srowen]

I'm sure the failure is spurious, so merged to master.
PS see https://spark.apache.org/contributing.html

[zoltanctoth]

@srowen Just about to submit a new doc relates pull request.
Wondering if your PS see https://spark.apache.org/contributing.html line referred to anything specific about how I should issue these PRs differently?

[srowen]

Minor stuff: we usually tag this with [MINOR] in the title to be clear there's no JIRA. Also ideal to batch together small related changes but I don't know that there was anything else to do here.