Fix a description about dimethyl labeling

Author: fcyu

docs/tutorial_silac.md

@@ -28,7 +28,7 @@ On the 'Quant (MS1)' tab, IonQuant has been set to run with SILAC light, medium,
 
 ![](https://raw.githubusercontent.com/Nesvilab/FragPipe/gh-pages/images/silac_4.png)
 
-If you are using light and heavy dimethyl labeling, change these fields to:
+If you are using light and heavy dimethyl labeling, adjust the variable modifications in the MSFragger tab and also change labeling settings to:
 
 ![](https://raw.githubusercontent.com/Nesvilab/FragPipe/gh-pages/images/silac_4_2.jpg)