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

contrib/initramfs/README.initramfs.markdown

@@ -1,9 +1,9 @@
 DESCRIPTION
-  These scripts is intended to be used with initramfs-tools, which is a similar
-  software product to "dracut" (which is more used in RedHat based distributions,
-  and is mainly used by Debian GNU/Linux and derivates to create a initramfs so
-  that the system can be booted of a ZFS filesystem. If you have no need or
-  interest for this, then it can safely be ignored.
+  These scripts are intended to be used with initramfs-tools, which is a similar
+  software product to "dracut" (which is used in RedHat based distributions),
+  and is mainly used by Debian GNU/Linux and derivatives to create an initramfs
+  so that the system can be booted off a ZFS filesystem. If you have no need or
+  interest in this, then it can safely be ignored.
 
   These script were written with the primary intention of being portable and
   usable on as many systems as possible.