Update 4_ZPFs.md typot

src/4_ZPFs.md

@@ -155,7 +155,8 @@ Coherent states have maximal "classical" energy, and even though they are (infin
 - The expectation values of observables of stationary states do not depend on time. 
 - However, there are quantum fluctuations. These are related to the spread/curvature of the wavefunction.
 - The coherent state is not a stationary state. Its time dynamics resembles that of a classical particle. 
-- The expectation values for position and momentum of the coherent state $|\alpha\rangle$ can expressed in terms of $\alpha$   
+- The coherent state is an eigenstate of the annihilation operator.
+- The expectation values for position and momentum of the coherent state $|\alpha\rangle$ can be expressed in terms of $\alpha$.   
 
 #  Homework Lecture 4