message: DOC: Fix crate feature mention ("flag" is redundant)
diff: //! which is backed by a hash table and the node identifiers are the keys
  //! into the table.
  //!
  -//! Optional crate feature flags: `"serde-1"`, see the Readme for more information.
  +//! Optional crate feature: `"serde-1"`, see the Readme for more information.
  //!
  #![doc(html_root_url = "https://docs.rs/petgraph/0.4/")]
  