Recode when there is part of string included

I have a dataset looks like this below.

df <- data.frame(id = c(1,2,3,4,5),
                 text = c("John S", "John P", "John","Steve","Sam"))

> df
  id   text
1  1 John S
2  2 John P
3  3   John
4  4  Steve
5  5    Sam

What I would like to do is to recode the values including John as `John' and keep the other same.

My desired output is below:

> df1
  id  text
1  1  John
2  2  John
3  3  John
4  4 Steve
5  5   Sam

Any ideas? Thanks