doc: fix a few typos in Writer::serialize

PR #220

src/writer.rs

@@ -777,7 +777,7 @@ impl<W: io::Write> Writer<W> {
  /// ## Non-Struct Containers
  ///
  /// Nested containers are flattened to their scalar components, with the
- /// exeption of a few types that are not allowed:
+ /// exception of a few types that are not allowed:
  ///
  /// | Name | Example Type | Example Value | Output |
  /// | ---- | ---- | ---- | ---- |
@@ -831,7 +831,7 @@ impl<W: io::Write> Writer<W> {
  /// ```
  ///
  /// However, if `has_headers` were enabled in the above example, then
- /// serialization would return an error. Speficially, when `has_headers` is
+ /// serialization would return an error. Specifically, when `has_headers` is
  /// `true`, there are two restrictions:
  ///
  /// 1. Named field values in structs must be scalars.