Improve phrasing in `docs/coordinates/index.rst`
Improve phrasing in `docs/coordinates/index.rst`

larrybradley/astropy,mhvk/astropy,pllim/astropy,larrybradley/astropy,lpsinger/astropy,pllim/astropy,larrybradley/astropy,pllim/astropy,astropy/astropy,astropy/astropy,lpsinger/astropy,lpsinger/astropy,mhvk/astropy,mhvk/astropy,mhvk/astropy,astropy/astropy,pllim/astropy,larrybradley/astropy,pllim/astropy,lpsinger/astropy,larrybradley/astropy,astropy/astropy,astropy/astropy,lpsinger/astropy,mhvk/astropy