Fix typo in documentation of Cursor::punct

"An Punct" here is left over from when Punct used to be named "an Op".
a74a1c8

src/buffer.rs

@@ -270,7 +270,7 @@ impl<'a> Cursor<'a> {
         }
     }
 
-    /// If the cursor is pointing at an `Punct`, returns it along with a cursor
+    /// If the cursor is pointing at a `Punct`, returns it along with a cursor
     /// pointing at the next `TokenTree`.
     pub fn punct(mut self) -> Option<(Punct, Cursor<'a>)> {
         self.ignore_none();