include/linux/lz4.h: fix spelling and copy-paste errors in documentation

Fix a few spelling and grammar errors, and two places where fast/safe in
the documentation did not match the function.

Link: http://lkml.kernel.org/r/20190321014452.13297-1-tomlevy93@gmail.com
Signed-off-by: Tom Levy <tomlevy93@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jiri Kosina <trivial@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>