Documentation: fix typo in root= kernel parameter description

Fixes a typo in the root= parameter description, changing
"this a a" to "this is a".

Fixes: c0c1a7d ("init: move the nfs/cifs/ram special cases out of name_to_dev_t")
Signed-off-by: Petr Vaněk <[email protected]>
Link: https://lore.kernel.org/[email protected]
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: arkamar

Documentation/admin-guide/kernel-parameters.txt

@@ -6253,7 +6253,7 @@
 			port and the regular usb controller gets disabled.
 
 	root=		[KNL] Root filesystem
-			Usually this a a block device specifier of some kind,
+			Usually this is a block device specifier of some kind,
 			see the early_lookup_bdev comment in
 			block/early-lookup.c for details.
 			Alternatively this can be "ram" for the legacy initial