quote the binary name in the warning

quote the binary name in the warning

to avoid confusion when the warning happens inside a workspace, example:
pwd is rustlearnage/rust_books/1_first_edition/closures/
and `cargo run` shows:

warning: path `src/main.rs` was erroneously implicitly accepted for binary match,
please set bin.path in Cargo.toml

(note that 'match' is not quoted, so it's confusing)

the 'match' project that it's referring to is in rustlearnage/rust_books/1_first_edition/match/
(note: Cargo.toml [workspace] is in rustlearnage/ )