Update README.md

Author: winkj

README.md

@@ -21,7 +21,7 @@ If you check the serial monitor window, you should see something like this:
 ## Serial Plotter (Graphical output)
 
 1. Open the example from ```File > Examples > arduino-sps > sps30```. 
-1. Open the serial plotter from the Tools menu
+1. Open the serial plotter from the Tools menu (if you have the serial monitor open, you'll need to close it first)
 1. Uncomment line 8 of the sample program; it should now read ```#define PLOTTER_FORMAT```
 1. Press the "Upload" button to compile and upload the sketch