bcm963xx_tag.h: fix duplicated word

Change doubled word "is" to "it is".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: bcm-kernel-feedback-list@broadcom.com
Cc: linux-mips@vger.kernel.org
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>