Rollup merge of #29787 - dbrgn:docs-fix, r=steveklabnik

Fix usage of wrong article in `Result` docs.

This is my first rust PR, if something about the process is wrong let me know.

As this is a documentation change, I believe the correct highfive line to use is the following:

r? @steveklabnik

src/libcore/result.rs

@@ -456,7 +456,7 @@ pub fn as_mut_slice(&mut self) -> &mut [T] {
     // Transforming contained values
     /////////////////////////////////////////////////////////////////////////
 
-    /// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to an
+    /// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to a
     /// contained `Ok` value, leaving an `Err` value untouched.
     ///
     /// This function can be used to compose the results of two functions.
@@ -484,7 +484,7 @@ pub fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U,E> {
         }
     }
 
-    /// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to an
+    /// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to a
     /// contained `Err` value, leaving an `Ok` value untouched.
     ///
     /// This function can be used to pass through a successful result while handling