Fix sign error

instructions/exercise_4.tex

@@ -229,7 +229,7 @@ in your command line/shell before executing your program.
 			\item Calculate the Hesse matrix once and store its value:
 			$$H_k= \nabla^2f(x_k)$$
 			\item Calculate the step size:
-			$$\alpha_k = \frac{\left\langle g_k, d_k\right\rangle}{\left\langle d_k, H_k \cdot d_k\right\rangle}$$
+			$$\alpha_k = -\frac{\left\langle g_k, d_k\right\rangle}{\left\langle d_k, H_k \cdot d_k\right\rangle}$$
 			\item Update the current position:
 			$$x_{k+1} = x_k +\alpha_k \cdot d_k$$
 			\item Calculate the next gradient: