Fix "Step definitions" example in README and improve pod name
Fix "Step definitions" example in README and improve pod name

This example now should compile and pod name is more Ruby style.
net-a-porter-mobile/XCTest-Gherkin,net-a-porter-mobile/XCTest-Gherkin,net-a-porter-mobile/XCTest-Gherkin,net-a-porter-mobile/XCTest-Gherkin