You can use regular expresions (regex), see this example:
library(tidyverse)
sample_df <- data.frame(
URL = c("football.html", "Athletics.html")
)
sample_df %>%
mutate(games = str_remove(URL, "\\.html$"))
#> URL games
#> 1 football.html football
#> 2 Athletics.html Athletics
Created on 2021-05-18 by the reprex package (v2.0.0)
Note: Next time please provide a proper REPRoducible EXample (reprex) illustrating your issue.