Minor whitespace changes (#2595)

src/references/shared.md

@@ -14,8 +14,10 @@ the referenced data cannot change.
 fn main() {
     let a = 'A';
     let b = 'B';
+
     let mut r: &char = &a;
     println!("r: {}", *r);
+
     r = &b;
     println!("r: {}", *r);
 }

src/references/strings.md

@@ -23,6 +23,7 @@ fn main() {
 
     let mut s2: String = String::from("Hello ");
     println!("s2: {s2}");
+
     s2.push_str(s1);
     println!("s2: {s2}");