Update 3_vector_spaces.md

src/3_vector_spaces.md

@@ -18,7 +18,7 @@ The lecture on vector spaces consists of two parts, each with their own video:
 -->
 **You can access the first video of this lecture [here](https://www.dropbox.com/s/evytrbb55fgrcze/linear_algebra_01.mov?dl=0)
 
-<iframe width="100%" height=315 src="https://www.dropbox.com/s/evytrbb55fgrcze/linear_algebra_01.mov?raw=1" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
+<iframe width="100%" height=315 src="https://www.dropbox.com/s/evytrbb55fgrcze/linear_algebra_01.mov?dl=0" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
 
 A vector $\vec{v}$ is essentially a mathematical object characterised by both
 a **magnitude** and a **direction**, that is, an orientation in a given space.