Merge pull request #2796 from Harmelodic/patch-1

Update ch03-03-how-functions-work.md: Pervasive -> Prevalent.

src/ch03-03-how-functions-work.md

@@ -1,6 +1,6 @@
 ## Functions
 
-Functions are pervasive in Rust code. You’ve already seen one of the most
+Functions are prevalent in Rust code. You’ve already seen one of the most
 important functions in the language: the `main` function, which is the entry
 point of many programs. You’ve also seen the `fn` keyword, which allows you to
 declare new functions.