Fix lint issues with projectID parameter name.
Fix lint issues with projectID parameter name.

Go style suggests that acronyms, including "ID", should have a consistent case.

> This rule also applies to "ID" when it is short for "identifier," so write
> "appID" instead of "appId".

Source: https://github.com/golang/go/wiki/CodeReviewComments#initialisms

This change fixes the following lint issues:

	github/projects.go:119:46: method parameter projectId should be projectID
	github/projects.go:176:47: method parameter projectId should be projectID

Updates #467.
Closes #498.

Change-Id: I53c5bd5ac59cad1b4ae955c8384ff7b25a20b278

google/go-github