You can argue it however the prosecutor can amend the charge to the proper statute. There are other legal arguments that can be made to keep this off your record. I would recommend talking with an attorney.
I'm not convinced that isn't the proper citation. There are traffic control devices stating you cannot drive in the HOV lane with less than two or three people (depending on where you are). You failed to obey that traffic control device. Therefore, you violated RCW 46.61.050.
As the other attorneys indicated, you can argue this yourself, but you probably shouldn't. There are many good attorneys in your area who handle traffic tickets. Depending on a number of factors, you could have an argument to make, but the attorneys have several more to make, and dramatically increase the odds of getting your ticket dismissed.
I definitely agree with my fellow counsel in that you may be able to argue that but there are many factors you should consider. First, what court is your ticket filed in? Also, do you know which Judge will be hearing it, and if there will be a prosecutor there that can change the statute to the correct one as Brad suggested? Does the officer's report mention that there is a sign indicating that this is an HOV lane or does he simply say you were in an HOV lane? If he doesn't mention a sign then your argument may be stronger because there is no evidence of a traffic control device.
These are just a few factors that I and my fellow attorneys who argue these tickets on a daily basis would consider, and it could be well worth the money to hire us to keep this off your record.