describe parameters can now be classes without error

Adding to_s on my_description allows my_description to be a class.

Before, the following code would create an error:
```Ruby
describe "An instance of", Dog do
  it "sleeps all day" do
    expect(@dog).to be_asleep?
  end
end
```
This is because it passed Dog to my_description and an could not be added to the string before it.