Update grammar in std::cell docs.

Using "having" in both the leading sentence and the bullets is unnecessary.
It makes it read as "it is only possible to have having several immutable...".

core/src/cell.rs

@@ -3,8 +3,8 @@
 //! Rust memory safety is based on this rule: Given an object `T`, it is only possible to
 //! have one of the following:
 //!
-//! - Having several immutable references (`&T`) to the object (also known as **aliasing**).
-//! - Having one mutable reference (`&mut T`) to the object (also known as **mutability**).
+//! - Several immutable references (`&T`) to the object (also known as **aliasing**).
+//! - One mutable reference (`&mut T`) to the object (also known as **mutability**).
 //!
 //! This is enforced by the Rust compiler. However, there are situations where this rule is not
 //! flexible enough. Sometimes it is required to have multiple references to an object and yet