Fix README.md rake command for running the tests
Fix README.md rake command for running the tests

So far, available tasks are:

```
rake install          # Build and install pendulum-0.1.1.gem into system gems
rake install:local    # Build and install pendulum-0.1.1.gem into system gems without network access
rake release[remote]  # Create tag v0.1.1 and build and push pendulum-0.1.1.gem to Rubygems
rake spec             # Run RSpec code examples
```
monochromegane/pendulum,monochromegane/pendulum