Rename `structure_sheaf` to `Spec.structure_sheaf`

Stream: general

[Jujian Zhang]

Structure sheaf of Spec is defined in src/algebraic_geometry/structure_sheaf.lean with name docs#algebraic_geometry.structure_sheaf. I think it should be put in a namespace for example algebraic_geometry.Spec.structure_sheaf as lots of other things will have structure sheaves as well. For example, I am currently working on moving Proj into mathlib and I need to talk about structure sheaf as well.