example should use -e ../lib/yard-rspec.rb (with .rb extension)

The readme suggested way to run run gives an error for me on mac:
```
% yardoc -e ../lib/yard-rspec example_code.rb 
[error]: The file `../lib/yard-rspec' could not be loaded:
```

I had to instead run:
```
% yardoc -e ../lib/yard-rspec.rb example_code.rb
```


