Update 101-vector.md

Added a video on the introduction to "Vectors" for game development".

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 # Vector
 
 `Vector` in game development is a mathematical concept and an integral part of game physics. It represents a quantity that has both magnitude and direction. A vector can be used to represent different elements in a game like positions, velocities, accelerations, or directions. In 3D games, it's commonly used to define 3D coordinates (x, y, z). For example, if you have a character in a game and you want to move it up, you'd apply a vector that points upward. Hence, understanding how to manipulate vectors is a fundamental skill in game development.
+- [Introduction to Vectors (By Sebastian Lague)](https://youtu.be/m7VY1T6f8Ak?feature=shared)