Tiny optimization to capture_haml

### WHAT

Reduce the amount of `Range` objects being created on this line to zero.

### WHY

When profiling my code, I noticed that this line created a few thousand `Range` objects per request. It's because for every line in the captured haml buffer, it created a range with `min_tabs..-1`. We can achieve the same result by using `slice(min_tabs, line.length)`.