fix doc comment of Length::is_fill

Author: edwloef

core/src/length.rs

@@ -37,8 +37,8 @@ impl Length {
         }
     }
 
-    /// Returns `true` iff the [`Length`] is either [`Length::Fill`] or
-    // [`Length::FillPortion`].
+    /// Returns `true` if the [`Length`] is either [`Length::Fill`] or
+    /// [`Length::FillPortion`].
     pub fn is_fill(&self) -> bool {
         self.fill_factor() != 0
     }