Initial fixes to `convolve_fft` examples.
Initial fixes to `convolve_fft` examples.

aleksandr-bakanov/astropy,pllim/astropy,StuartLittlefair/astropy,astropy/astropy,saimn/astropy,mhvk/astropy,lpsinger/astropy,lpsinger/astropy,saimn/astropy,mhvk/astropy,StuartLittlefair/astropy,astropy/astropy,StuartLittlefair/astropy,StuartLittlefair/astropy,pllim/astropy,saimn/astropy,aleksandr-bakanov/astropy,mhvk/astropy,larrybradley/astropy,larrybradley/astropy,lpsinger/astropy,pllim/astropy,larrybradley/astropy,mhvk/astropy,saimn/astropy,lpsinger/astropy,larrybradley/astropy,mhvk/astropy,StuartLittlefair/astropy,astropy/astropy,saimn/astropy,aleksandr-bakanov/astropy,pllim/astropy,pllim/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,astropy/astropy,astropy/astropy,larrybradley/astropy