docs: cleanup the explanation paragraphs at the start of each exercise.

exercises/strings/strings4.rs

@@ -1,9 +1,10 @@
 // strings4.rs
-
+//
 // Ok, here are a bunch of values-- some are `String`s, some are `&str`s. Your
 // task is to call one of these two functions on each value depending on what
 // you think each value is. That is, add either `string_slice` or `string`
 // before the parentheses on each line. If you're right, it will compile!
+//
 // No hints this time!
 
 // I AM NOT DONE