Update 6_backaction.md

src/6_backaction.md

@@ -64,7 +64,7 @@ What does continuous measurement even mean?  Surely we cannot just continuously
 
 So, if we are looking "all the time", how often does the wavefunction collapse?
 
-1. The collapse occurs randomly in time
+1. The collapse occurs randomly in time.
 
 2. It happens at an average "collapse rate" determined by how quickly I obtain information about the thing I am observing