Update README.initramfs.markdown
Fix several typos and grammar.
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Arno van Wyk <avw1987@users.noreply.github.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7080