Use a semicolon instead of an empty options list in example .proto files

[chalin]

Originally from @waldyrious via #184 (comment)

Use a semicolon instead of an empty options list.

This is consistent with the style guide and examples and in the protobuf documentation:

• https://developers.google.com/protocol-buffers/docs/style#services
• https://developers.google.com/protocol-buffers/docs/proto3#services
• https://developers.google.com/protocol-buffers/docs/reference/proto3-spec#service_definition