Fix typo in cell.rs

src/libcore/cell.rs

@@ -1256,7 +1256,7 @@ fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 /// To assist with proper design, the following scenarios are explicitly declared legal
 /// for single-threaded code:
 ///
-/// 1. A `&T` reference can be released to safe code and there it can co-exit with other `&T`
+/// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
 /// references, but not with a `&mut T`
 ///
 /// 2. A `&mut T` reference may be released to safe code, provided neither other `&mut T` nor `&T`