Update 4_vector_spaces_QM.md

src/4_vector_spaces_QM.md

@@ -18,6 +18,9 @@ The contents of this lecture are summarised in the following **videos**:
 
 - [4_vector_spaces_QM_video1](https://www.dropbox.com/s/mnccmpff33pre9r/linear_algebra_04.mov?dl=0)
 
+- [4_vector_spaces_QM_video2](https://www.dropbox.com/s/709bh9j083y7d0s/linear_algebra-06.mov?dl=0)
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 ## Dirac notation and Hilbert spaces
 
 In the previous lecture we reviewed the basic properties of linear vector spaces. We now discuss how the same formalism