Update 6_bonds_and_spectra.md

src/6_bonds_and_spectra.md

@@ -100,7 +100,7 @@ Our plan:
 * Write down equations of motion
 * Compute normal modes
 
-For simplicity we consider 1D motion, and let's start with 3 atoms:
+For simplicity we consider 1D motion, and let's start with 4 atoms:
 
 ![](figures/phonons2.svg)