fix 'let' regex syntax highlight to avoid capturing words like 'letter'

I came across what seems to be a bug in the syntax highlight regex. Before this commit this package would not only highlight:
```rspec
let
let!
```
but also part of words like ```letter``` 
I have updated the regex to fix the issue. It works but I am not highly proficient in regex so please excuse me if it's not the most elegant form.

Let me know what you see.