Thats fine, thats not a true watermark and will go away when you unroot. The **Unlocked** is the watermark
Mine shows revolutionary at the top as well and I have never touched the HTC dev method
To be fair, by definition, the text -Revolutionary- at the top of HBOOT is in fact a digital watermark. It is just a different watermark than the one left by the HTCDev unlocking process. In fact, the locked 2.18 stock HBOOT also has a digital watermark ("LOCKED (OOW)"), that allows Sprint/HTC to easily tell whether the phone has been unlocked.
As Rxpert said, the unrooting method for Revolutionary root will remove the Revolutionary watermark completely.
Using HTCDev.com to "relock" the 2.18 HBOOT however, will leave a watermark that says "RELOCKED" on HBOOT, which will alert Sprint/HTC that the device had been unlocked, and thus warranty has been voided.
While I'm sure there are a few EVOs that still qualify for the manufacturer's warranty (only a year old?), I think most are out of warranty anyway, so the watermark is really irrelevant. If you have Total Equipment Protection on your Sprint account/line, it doesn't matter if you return your device rooted or not. In fact, you're basically paying for rooting protection, in the even something happens to your device (of course, the deductible will apply if the phone has to be replaced).