Update 4_vector_spaces_QM.md

src/4_vector_spaces_QM.md

@@ -14,6 +14,8 @@ The lecture on vector spaces in quantum mechanics consists of the following part
 
 - [A two-dimensional Hilbert space](#two-dimensional-hilbert-space)
 
+and at the end of the lecture one can find the corresponding [Problems](#problems)
+
 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)
@@ -231,9 +233,7 @@ $$
 $$
 as required to ensure the probabilistic description of the state vector.
 
-
-Problems
-========
+## Problems
 
 **1)** [:grinning:] *The inner product*