Use lower-cased x-go-name for parameters var name

[emilekm]

Small change to how variable names are generated (a follow up to a comment on my previous MR and extension to it).

components:
  parameters:
    PetID:
      name: pet_id
      x-go-name: PetID
      in: path
      schema:
        type: string
      required: true

Before:

func (c *ClientWithResponses) UpdatePetWithResponse(ctx context.Context, petId PetID,  ...)

Now:

func (c *ClientWithResponses) UpdatePetWithResponse(ctx context.Context, petID PetID,  ...)

[emilekm]

Breaks compatibility, I'll look into doing this differently.