Fix doctest.

Author: calder

library/core/src/primitive_docs.rs

@@ -1335,14 +1335,16 @@ mod prim_f16 {}
 /// For example:
 ///
 /// ```
-/// x = x.algebraic_add(x, a.algebraic_mul(b));
-/// ```
+/// let x = 12.0;
+/// let a = 34.0;
+/// let b = 56.0;
 ///
-/// May be rewritten as either:
+/// // This:
+/// let x = x.algebraic_add(x, a.algebraic_mul(b));
 ///
-/// ```
-/// x = x + (a * b); // As written
-/// x = (a * b) + x; // Reordered to allow using a single `fma`
+/// // May be rewritten as either:
+/// let x = x + (a * b); // As written
+/// let x = (a * b) + x; // Reordered to allow using a single `fma`
 /// ```
 
 #[stable(feature = "rust1", since = "1.0.0")]