[DOCUMENTATION][KAFKA] fix typo in kafka/OffsetRange.scala

[microwishing]

this is to fix some typo in external/kafka/src/main/scala/org/apache/spark/streaming/kafka/OffsetRange.scala

[srowen]

@microwishing can you have a look at the rest of the doc around this for any other typos? even small PRs have nonzero overhead, so I'm encouraging people to not just open a PR for single typos as they see them, but take a little time, if they're bothering, to evaluate similar issues around the change. Maybe there aren't any.

[microwishing]

@srowen thanks for your tips. I happened to work on some kafka project recently so I went through source codes including comments for spark kafka streaming part today.

I spent some time setting up the spark dev environment and feel good so far due to rich doc. Those are indeed trivial typos. I can make check-ins next time after fixing some real bugs here.

[SparkQA]

Test build #2163 has finished for PR 10121 at commit 97dfb8b.

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

[srowen]

Merged to master/1.6