Fix another error in simple_math example

Author: jturner314

src/doc/ndarray_for_numpy_users/simple_math.rs

@@ -79,8 +79,8 @@
 //! // Add b to a (broadcasting to rows).
 //! let c = a + &b;
 //!
-//! // Matrix product of c transpose with a.
-//! let d = c.t().dot(&a);
+//! // Matrix product of c transpose with c.
+//! let d = c.t().dot(&c);
 //! # }
 //! ```
 //!