Corrected links/usernames in performance guide
Corrected links/usernames in performance guide

This fixes two broken links in the performance guide and removes the
mention of Josh as he no longer works for GitLab.

[ci skip]

t-zuehlsdorff/gitlabhq,jirutka/gitlabhq,LUMC/gitlabhq,iiet/iiet-git,mmkassem/gitlabhq,allysonbarros/gitlabhq,dreampet/gitlab,htve/GitlabForChinese,shinexiao/gitlabhq,dplarson/gitlabhq,stoplightio/gitlabhq,axilleas/gitlabhq,stoplightio/gitlabhq,SVArago/gitlabhq,iiet/iiet-git,daiyu/gitlab-zh,screenpages/gitlabhq,shinexiao/gitlabhq,iiet/iiet-git,mr-dxdy/gitlabhq,icedwater/gitlabhq,LUMC/gitlabhq,darkrasid/gitlabhq,LUMC/gitlabhq,t-zuehlsdorff/gitlabhq,axilleas/gitlabhq,mmkassem/gitlabhq,openwide-java/gitlabhq,icedwater/gitlabhq,daiyu/gitlab-zh,shinexiao/gitlabhq,openwide-java/gitlabhq,allysonbarros/gitlabhq,axilleas/gitlabhq,daiyu/gitlab-zh,dplarson/gitlabhq,mr-dxdy/gitlabhq,daiyu/gitlab-zh,jirutka/gitlabhq,shinexiao/gitlabhq,allysonbarros/gitlabhq,icedwater/gitlabhq,htve/GitlabForChinese,dplarson/gitlabhq,dreampet/gitlab,htve/GitlabForChinese,openwide-java/gitlabhq,darkrasid/gitlabhq,dreampet/gitlab,htve/GitlabForChinese,icedwater/gitlabhq,axilleas/gitlabhq,openwide-java/gitlabhq,darkrasid/gitlabhq,screenpages/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq,stoplightio/gitlabhq,t-zuehlsdorff/gitlabhq,darkrasid/gitlabhq,mr-dxdy/gitlabhq,SVArago/gitlabhq,allysonbarros/gitlabhq,LUMC/gitlabhq,jirutka/gitlabhq,iiet/iiet-git,t-zuehlsdorff/gitlabhq,mmkassem/gitlabhq,SVArago/gitlabhq,jirutka/gitlabhq,dreampet/gitlab,mr-dxdy/gitlabhq,screenpages/gitlabhq,screenpages/gitlabhq,dplarson/gitlabhq,SVArago/gitlabhq