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

exercises/lifetimes/lifetimes1.rs

@@ -1,11 +1,12 @@
 // lifetimes1.rs
 //
 // The Rust compiler needs to know how to check whether supplied references are
-// valid, so that it can let the programmer know if a reference is at risk
-// of going out of scope before it is used. Remember, references are borrows
-// and do not own their own data. What if their owner goes out of scope?
+// valid, so that it can let the programmer know if a reference is at risk of
+// going out of scope before it is used. Remember, references are borrows and do
+// not own their own data. What if their owner goes out of scope?
 //
-// Execute `rustlings hint lifetimes1` or use the `hint` watch subcommand for a hint.
+// Execute `rustlings hint lifetimes1` or use the `hint` watch subcommand for a
+// hint.
 
 // I AM NOT DONE
 

exercises/lifetimes/lifetimes2.rs

@@ -1,10 +1,10 @@
 // lifetimes2.rs
 //
-// So if the compiler is just validating the references passed
-// to the annotated parameters and the return type, what do
-// we need to change?
+// So if the compiler is just validating the references passed to the annotated
+// parameters and the return type, what do we need to change?
 //
-// Execute `rustlings hint lifetimes2` or use the `hint` watch subcommand for a hint.
+// Execute `rustlings hint lifetimes2` or use the `hint` watch subcommand for a
+// hint.
 
 // I AM NOT DONE
 

exercises/lifetimes/lifetimes3.rs

@@ -2,7 +2,8 @@
 //
 // Lifetimes are also needed when structs hold references.
 //
-// Execute `rustlings hint lifetimes3` or use the `hint` watch subcommand for a hint.
+// Execute `rustlings hint lifetimes3` or use the `hint` watch subcommand for a
+// hint.
 
 // I AM NOT DONE