Update E0423 description
Update E0423 description

E0423 doesn't apply only to structs, update the error index description
to make this clear.

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