Constants.h: Fix possible typo in r216015. [-Wdocumentation]

  /// Getelementptr form.  Value* is only accepted for convenience;

  /// all elements must be Constant's.

  ///

  /// \param OnlyIfReduced see \a getWithOperands() docs.

  /// \param OnlyIfReducedTy see \a getWithOperands() docs.

  static Constant *getGetElementPtr(Constant *C, ArrayRef<Constant *> IdxList,

                                    bool InBounds = false,

                                    Type *OnlyIfReducedTy = nullptr) {