fixed typo

src/6_backaction.md

@@ -93,7 +93,7 @@ We have talked previously about laser interferometers and the sensitivity one wo
 One of the striking features of QM is the prediction that even if we cool a vibrating object to absolute zero, it will still be "moving" due to zero-point fluctuations.  As we saw earlier, one easy way to remember how big are these fluctuations is to apply the equipartition theorem but replace $\frac{1}{2}kT$ with $\frac{1}{2}E_0 = \frac{1}{4}\hbar\omega$:
 
 $$
-\frac{1}{2}m\omega^2 x_{ZPF} = \frac{1}{4}\hbar\omega \rightarrow x_{ZPF} = \sqrt{\frac{\hbar}{2m\omega}}
+\frac{1}{2}m\omega^2 x^2_{ZPF} = \frac{1}{4}\hbar\omega \rightarrow x_{ZPF} = \sqrt{\frac{\hbar}{2m\omega}}
 $$
 
 Now, as experimentalists, we might ask whether we can build something to measure this.