use next() instead of nth(0) in exercise examples (#1018)

Author: coriolinus

exercises/dot-dsl/example.rs

@@ -40,7 +40,7 @@ pub mod graph {
         }
 
         pub fn get_node(&self, name: &str) -> Option<&Node> {
-            self.nodes.iter().filter(|n| n.name == name).nth(0)
+            self.nodes.iter().filter(|n| n.name == name).next()
         }
     }
 

exercises/phone-number/example.rs

@@ -5,7 +5,7 @@ pub fn number(user_number: &str) -> Option<String> {
 
     if number_len < 10
         || number_len > 11
-        || (filtered_number.len() == 11 && filtered_number.chars().nth(0).unwrap() != '1')
+        || (filtered_number.len() == 11 && filtered_number.chars().next().unwrap() != '1')
     {
         return None;
     }
@@ -16,7 +16,7 @@ pub fn number(user_number: &str) -> Option<String> {
 
     if filtered_number
         .chars()
-        .nth(0)
+        .next()
         .unwrap()
         .to_digit(10)
         .unwrap()