Get rid of related videos

src/1_complex_numbers.md

@@ -14,7 +14,7 @@ The lecture on complex numbers consists of three parts, each with their own vide
 
 ## Definition and basic operations
 
-<iframe width="100%" height=315 src="https://www.youtube-nocookie.com/embed/fLMdaMuEp8s" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
+<iframe width="100%" height=315 src="https://www.youtube-nocookie.com/embed/fLMdaMuEp8s?rel=0" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
 
 Complex numbers are numbers of the form $$z = a + b {\rm i}.$$ Here
 $\rm i$ is the square root of -1: $${\rm i} = \sqrt{-1},$$ or,