Fix duplicated words in documentation

[latent-9]

A few duplicated words in the docs prose:

• docs/internals.rst: "into the the metadata_* table" -> "into the metadata_* table" (4 parallel method docstrings: instance/databases/resources/columns).
• docs/plugin_hooks.rst: "those values will will be merged" -> "will be merged"; and "it can return return an awaitable" -> "return an awaitable".
• docs/contributing.rst: "you can use use sphinx-autobuild" -> "you can use".

Documentation only.