Fix documentation and some comments (article, grammar)

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
diff --git a/qemu-doc.texi b/qemu-doc.texi
index 0b2746f..3840585 100644
--- a/qemu-doc.texi
+++ b/qemu-doc.texi
@@ -1037,7 +1037,7 @@
 @node disk_images_gluster
 @subsection GlusterFS disk images
 
-GlusterFS is an user space distributed file system.
+GlusterFS is a user space distributed file system.
 
 You can boot from the GlusterFS disk image with the command:
 @example