Remove "kernels can be excluded" from README.md (#3329)

Fixed a small typo, reworded a paragraph, and then removed "(kernels can be excluded)" from the description of how to download data for each mission. It was present for _each mission_, so including (kernels can be excluded) didn't communicate any real, mission-specific information. The paragraph above the mission-specific data downloads explains that the kernels can be excluded unless a mission-specific calibration program or end-to-end program needs them and directs the reader to consult mission-specific documentation.