fix github strategy is_valid method
fix github strategy is_valid method

project-anacapa/course-github-org-tool,project-anacapa/course-github-org-tool,UCSB-CS-Using-GitHub-In-Courses/course-github-org-tool,project-anacapa/course-github-org-tool,project-anacapa/course-github-org-tool,UCSB-CS-Using-GitHub-In-Courses/course-github-org-tool,UCSB-CS-Using-GitHub-In-Courses/course-github-org-tool