minor: style fix, removing redudant reference to self

The style checker is currently failing due to a unnecessary references to self
in Scope#inspect. This commit resolves that failure.

```bash mongo-ruby-driver|master ⇒ bundle update mongo-ruby-driver|master ⇒
rubocop --version 0.10.0 mongo-ruby-driver|master ⇒ rubocop Inspecting 30
files
............C.................

Offences:

lib/mongo/scope.rb:83:26: C: Redundant `self` detected.
     "<Mongo::Scope:0x#{self.object_id} " +
                        ^^^^
```