Naming fixes

[dakota002]

Naming fixes for example files

[Vidushi-GitHub]

We want to keep the convention as mentioned: https://gcn.nasa.gov/docs/notices/schema.
Thus it should be lower-case instead.

"Mission name should be lowercase and the schema name should be snake_case. Each file corresponds to a Kafka topic named gcn.notices.mission.schema_name."

[Vidushi-GitHub]

I have separated these modifications into two different PRs,
as both are for different teams.

Please see:
Merged: #148
In Review: https://github.com/nasa-gcn/gcn-schema/pull/149/files.