fix(doc): fix typos in comments on design
// Current design:
// There are four main types: Value, Table, ArrayOfTables and Document(Inner).
// Document allocates all tables in the typed arena.
-// Document also contains an intrusive list off all documents.
+// Document also contains an intrusive list of all tables.
// Each table contains a link to previous and next table,
// the order is same as in the toml string representation.
// Also each table and array of tables contain a link to the document,
