Fix slightly broken link to TOML in "The Cargo Book" section 3.2.1 Cargo Targets

Problem

The link to "array-of-table of TOML" in section 3.2.1 "Cargo Targets" (markdown file) is somewhat broken. It attempts to visit the hashed URL: https://github.com/toml-lang/toml#array-of-tables, but no element with id="array-of-tables" can be found on that page. I believe the desired resource now lives here: https://github.com/toml-lang/toml/blob/master/toml.md#array-of-tables

Possible Solution(s)

Update the link from https://github.com/toml-lang/toml#array-of-tables to https://github.com/toml-lang/toml/blob/master/toml.md#array-of-tables