Add missing code voice.

Author: amartini51

stdlib/public/core/FloatingPointTypes.swift.gyb

@@ -94,7 +94,7 @@ extension ${Self}: CustomStringConvertible {
   /// A textual representation of the value.
   ///
   /// For all finite values, the value of this property is a string that can be
-  /// converted back to an instance of ${Self} without rounding errors.  That
+  /// converted back to an instance of `${Self}` without rounding errors.  That
   /// is, if `x` is an instance of `${Self}`, then `${Self}(x.description) ==
   /// x` is always true.  For any NaN value, the property's value is "nan", and
   /// for positive and negative infinity its value is "inf" and "-inf".