auto merge of #16775 : jbcrail/rust/fix-rational-docs, r=pcwalton

Minor corrections to Rational documentation.

src/libnum/rational.rs

@@ -154,7 +154,7 @@ pub fn trunc(&self) -> Ratio<T> {
         Ratio::from_integer(self.numer / self.denom)
     }
 
-    ///Returns the fractional part of a number.
+    /// Returns the fractional part of a number.
     #[inline]
     pub fn fract(&self) -> Ratio<T> {
         Ratio::new_raw(self.numer % self.denom, self.denom.clone())
@@ -243,7 +243,7 @@ fn $method(&self, rhs: &Ratio<T>) -> Ratio<T> {
     }
 }
 
-// a/b + c/d = (a*d + b*c)/(b*d
+// a/b + c/d = (a*d + b*c)/(b*d)
 arith_impl!(impl Add, add)
 
 // a/b - c/d = (a*d - b*c)/(b*d)