Update introduction.md

[csprasad]

Removed Unused Declaration of Variable speed.

I noticed that the variable speed is declared but never used in the code example. To avoid confusion, I suggest removing this unused declaration.

[BNAndras]

Seems like speed is used later in myChar.stats = CharacterStats(health: 72.8, speed: 19, strength: 6). So you'll need to edit that bit later on as well.

[meatball133]

I don't really see how this adds confusion, since even though if not used it can still be important for the codebase itself, thereby I wouldnt strictly speaking say that it should be removed