Mark highlight_dynamic_prompt as deprecated

And fix example.
diff --git a/examples/example.rs b/examples/example.rs
index ce41cf4..e02f665 100644
--- a/examples/example.rs
+++ b/examples/example.rs
@@ -35,8 +35,12 @@
 }
 
 impl Highlighter for MyHelper {
-    fn highlight_prompt<'p>(&self, _: &str) -> Cow<'static, str> {
-        Borrowed(COLORED_PROMPT)
+    fn highlight_prompt<'p>(&self, prompt: &'p str) -> Cow<'p, str> {
+        if prompt == PROMPT {
+            Borrowed(COLORED_PROMPT)
+        } else {
+            Borrowed(prompt)
+        }
     }
 
     fn highlight_hint<'h>(&self, hint: &'h str) -> Cow<'h, str> {
diff --git a/src/highlight.rs b/src/highlight.rs
index c09e5e1..7ecba81 100644
--- a/src/highlight.rs
+++ b/src/highlight.rs
@@ -26,6 +26,7 @@
     }
     /// Takes the dynamic `prompt` and
     /// returns the highlighted version (with ANSI color).
+    #[deprecated(since="2.0.1", note="please use `highlight_prompt` instead")]
     fn highlight_dynamic_prompt<'p>(&self, prompt: &'p str) -> Cow<'p, str> {
         Borrowed(prompt)
     }