minor: style fix, removing redudant reference to self

The style checker is currently failing due to an unnecessary reference to `self`
in Scope#inspect. This commit resolves that.

```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} " +
                        ^^^^
```