Update build manual for no foil knobs

Signed-off-by: Petr Horacek <[email protected]>

manual/build/manual.tex

@@ -168,7 +168,7 @@ \section{Knobs}
 You can now put knobs on the potentiometers.
 
 \begin{enumerate}
-  \item The silver part of the big knob is protected with a sticky foil. Peel it off.
+  \item If the silver part of the big knob is protected with a sticky foil, peel it off.
   \item Put the big knob on the topmost potentiometer. Use a small flat screwdriver to tighten the screw of the pot. The screw should be pressing against the flat part of the shaft. Keep some distance between the knob and the panel.
   \item Put smaller black knobs on the remaining pots. Align them with the D-shaped shaft and press them in. You may need to pull them a little bit if you see they are scratching the nut.
 \end{enumerate}