docs/push-notifications: Make some trivial fixes.
docs/push-notifications: Make some trivial fixes.

When these instructions were first written, it was true that you'd
be modifying Java code to work client-side on Android notifications.
Now it's Kotlin.

Also fix some confusing spacing (caused by mixed tabs and spaces!
don't know how those got there) in an example command.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile