message: fix typo in index_meta.rs
diff: @@ -213,7 +213,7 @@ pub struct IndexMeta {
  #[serde(skip_serializing_if = "Option::is_none")]
  /// Payload associated to the last commit.
  ///
  - /// Upon commit, clients can optionally add a small `Striing` payload to their commit
  + /// Upon commit, clients can optionally add a small `String` payload to their commit
  /// to help identify this commit.
  /// This payload is entirely unused by tantivy.
  pub payload: Option<String>,
  