Correct Rake-related typos in autoload/rails.vim

I got syntax errors while running :Rake in a test file:

```
Error detected while processing function <SNR>40_Rake..<SNR>40_readable_default_rake_task:
E121: Undefined variable: meth
E15: Invalid expression: meth =~ '^test_'
```

It looks like some typos were introduced in commit ebcc11.