Fix possessive "its" in docs and comments (#3998)

This PR fixes a number of places where we were incorrectly using "it's"
where we needed to use the possessive "its".

Release Notes:

- N/A

crates/rope/src/rope.rs

@@ -78,7 +78,7 @@ impl Rope {
     }
 
     pub fn slice_rows(&self, range: Range<u32>) -> Rope {
-        //This would be more efficient with a forward advance after the first, but it's fine
+        // This would be more efficient with a forward advance after the first, but it's fine.
         let start = self.point_to_offset(Point::new(range.start, 0));
         let end = self.point_to_offset(Point::new(range.end, 0));
         self.slice(start..end)