fixed error in deriviate

cheat-sheets/analysis/Analysis_Wichtige_Formeln.tex

@@ -71,7 +71,7 @@ e^x &= \sum_{n = 0}^{\infty} \frac {x^n}{n!} \\
 	(\arctan x)' &= \frac {1}{1 + x^2} \\
 	(\sin x)'    &= \cos x \\
 	(\cos x)'    &= -\sin x \\
-	(\arctanh x)' &= \frac {1}{1 + x^2}
+	(\arctanh x)' &= \frac {1}{1 - x^2}
 \end{align*}