fix doc indentation

src/lib.rs

@@ -44,11 +44,11 @@
 //! * [Event] fields are inserted as [GELF] additional fields, `_field_name`.
 //! * [Event] field named `message` is renamed to `short_message`.
 //! * If `short_message` (or `message`) [Event] field is missing then `short_message` is
-//! set to the empty string.
+//!      set to the empty string.
 //! * [Event] fields whose names collide with [GELF] required fields are coerced
-//! into the required types and overrides defaults given in the builder.
+//!     into the required types and overrides defaults given in the builder.
 //! * The hierarchy of spans is concatenated and inserted as `span_a:span_b:span_c` and
-//! inserted as an additional field `_span`.
+//!     inserted as an additional field `_span`.
 //!
 //! [GELF]: https://docs.graylog.org/en/3.1/pages/gelf.html
 //! [GELF format]: https://docs.graylog.org/en/3.1/pages/gelf.html