outlives/env: fix comment, say must and not should
outlives/env: fix comment, say must and not should

aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust