refactor(documentation/fundamentals): small stylistic change
refactor(documentation/fundamentals): small stylistic change

leanprover/leanprover.github.io,avigad/leanprover.github.io,leanprover/leanprover.github.io,avigad/leanprover.github.io,leanprover/leanprover.github.io,avigad/leanprover.github.io