Yes that is mild nutrient burn, feed your next couple times half strength of what you did last time. Try to get some run off each time you water the plants with nutrients. You can actually use that tip burn as an indicator for how strong to feed the plants, just always remember every plant can eat differently and then from there just feed accordingly. Awesome work.
Once the tips burn back your feed off 10% or so and ride that through until the last week of flower when you should just give plain water.