Fix a couple docs typos

src/libcore/cell.rs

@@ -11,7 +11,7 @@
 //! mutate it.
 //!
 //! Shareable mutable containers exist to permit mutability in a controlled manner, even in the
-//! presence of aliasing. Both `Cell<T>` and `RefCell<T>` allows to do this in a single threaded
+//! presence of aliasing. Both `Cell<T>` and `RefCell<T>` allow doing this in a single-threaded
 //! way. However, neither `Cell<T>` nor `RefCell<T>` are thread safe (they do not implement
 //! `Sync`). If you need to do aliasing and mutation between multiple threads it is possible to
 //! use [`Mutex`](../../std/sync/struct.Mutex.html),

src/libstd/env.rs

@@ -465,7 +465,7 @@ pub struct JoinPathsError {
 /// # }
 /// ```
 ///
-/// Using `env::join_paths` with `env::spit_paths` to append an item to the `PATH` environment
+/// Using `env::join_paths` with [`env::split_paths`] to append an item to the `PATH` environment
 /// variable:
 ///
 /// ```
@@ -483,6 +483,8 @@ pub struct JoinPathsError {
 ///     Ok(())
 /// }
 /// ```
+///
+/// [`env::split_paths`]: fn.split_paths.html
 #[stable(feature = "env", since = "1.0.0")]
 pub fn join_paths<I, T>(paths: I) -> Result<OsString, JoinPathsError>
     where I: IntoIterator<Item=T>, T: AsRef<OsStr>