chore: fix typo (#513)

src/widgets/mod.rs

@@ -62,8 +62,8 @@ bitflags! {
 
 /// Base requirements for a Widget
 pub trait Widget {
-    /// Draws the current state of the widget in the given buffer. That the only method required to
-    /// implement a custom widget.
+    /// Draws the current state of the widget in the given buffer. That is the only method required
+    /// to implement a custom widget.
     fn render(self, area: Rect, buf: &mut Buffer);
 }