Those two lines do not rename the columns, but create vectors called AveragePerf and Average. To check, look at names(Practice_performance_measures) after that line and the names will not have changed. Also if using RStudio, in the environment a vector called Average will appear. To rename the columns of the data set, I typically use the tidyverse such as:
library(tidyverse)
Practice_performance_measures <- Practice_performance_measures %>%
rename(Average = `Average Perf`)
without the tidyverse you could do something like this:
names(Practice_performance_measures)[which(names(Practice_performance_measures)=="Average Perf")] <- c("Average")