[Minor] Fix a typo of type parameter in JavaUtils.scala

[sarutak]

In JavaUtils.scala, thare is a typo of type parameter. In addition, the type information is removed at the time of compile by erasure.

This issue is really minor so I don't file in JIRA.

[srowen]

Should this really be A to express the relation to the generic bound? although underlying must already have keys of type A already. It just looks like that was the intent.

[sarutak]

I thought it should be A but the type parameter at the position is no mean because the erasure removes type information at the time of compile.
Even though, should we place A for readability?

[SparkQA]

Test build #24780 has finished for PR 3789 at commit 99f6f63.

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

[SparkQA]

Test build #24796 has finished for PR 3789 at commit e20193d.

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

[rxin]

Merging in master. Thanks.