DOC: Replace defunct link for Git tutorial
DOC: Replace defunct link for Git tutorial

lpsinger/astropy,saimn/astropy,lpsinger/astropy,pllim/astropy,larrybradley/astropy,saimn/astropy,mhvk/astropy,astropy/astropy,larrybradley/astropy,StuartLittlefair/astropy,lpsinger/astropy,pllim/astropy,mhvk/astropy,mhvk/astropy,larrybradley/astropy,pllim/astropy,larrybradley/astropy,mhvk/astropy,saimn/astropy,StuartLittlefair/astropy,astropy/astropy,astropy/astropy,StuartLittlefair/astropy,saimn/astropy,StuartLittlefair/astropy,mhvk/astropy,astropy/astropy,pllim/astropy,StuartLittlefair/astropy,lpsinger/astropy,larrybradley/astropy,pllim/astropy,lpsinger/astropy,astropy/astropy,saimn/astropy