That's an encoding error. When you cut and paste from Discourse (or anything on the internet, really) sometimes you'll get auto-formatting on the characters that you don't want (in this case, I'm guessing it's the quotation marks, which go from " to "word" (see how they're curly?) automatically on here (so, when you paste them they essentially read as “ and ” rather than ".
Short of the long— type the statement in directly!
brfss2013 %>%
count(income2, genhlth) %>%
group_by(income2) %>%
mutate(prop = n / sum(n)) %>%
filter(income2 == "Less than $10,000" | income2 == "Less than $50,000")
Here's a picture of the difference when rendered on here: