You only have to add
group = 1 into the ggplot or geom_line aes().
For line graphs, the data points must be grouped so that it knows which points to connect. In this case, it is simple — all points should be connected, so group=1. When more variables are used and multiple lines are drawn, the grouping for lines is usually done by variable.
plot5 <- ggplot(df, aes(year, pollution, group = 1)) + geom_point() + geom_line() + labs(x = "Year", y = "Particulate matter emissions (tons)", title = "Motor vehicle emissions in Baltimore")