Update imports.md
Update imports.md

Without the additional braces making the `SimpleRef.` optional in `ImportExpr`, an expression such as `import java as j` wouldn't technically be accepted by the grammar specification.

Co-authored-by: keynmol
lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,sjrd/dotty,lampepfl/dotty,lampepfl/dotty,sjrd/dotty,dotty-staging/dotty,dotty-staging/dotty,sjrd/dotty,lampepfl/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty