dot-dsl: Replace filter(p).next() with find(p) (#1024)

clippy::filter_next
https://rust-lang.github.io/rust-clippy/master/index.html#filter_next

Helps address #1012

Author: petertseng

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).next()
+            self.nodes.iter().find(|n| n.name == name)
         }
     }