Fix typo in Guards.md (#8142)
Fix typo in Guards.md (#8142)

`not not` -> `not`
ggcampinho/elixir,pedrosnk/elixir,kimshrier/elixir,kelvinst/elixir,kelvinst/elixir,joshprice/elixir,ggcampinho/elixir,pedrosnk/elixir,elixir-lang/elixir,lexmag/elixir,lexmag/elixir,kimshrier/elixir,michalmuskala/elixir