Fix small errors in primer.md
Fix small errors in primer.md

antidotcb/googletest,jlanecox/googletest,empiredan/googletest,jlanecox/googletest,google/googletest,jlanecox/googletest,google/googletest,empiredan/googletest,gianricardo/googletest,jlanecox/googletest,gianricardo/googletest,empiredan/googletest,google/googletest,gianricardo/googletest,antidotcb/googletest,antidotcb/googletest,google/googletest,antidotcb/googletest