Update compiler.rs

[Gudnessuche]

made comment more explanatory

[notmandatory]

As mentioned in your other PR I suggest you start with something a little more substantial. Minor grammar and punctuation fixes aren't a good way to start contributing to a project. It's better to find an issue where you can demonstrate some knowledge of bitcoin or the code base, such as improving the substance of documentation, testing, or fixing some small issue even if it's considered low priority.