Fix gradle plugin version in docs

[gosar]

Background

Noticed on https://smithy.io/2.0/guides/gradle-plugin/index.html that the version of gradle plugin is 0.20.0. Was expecting to see the new 1.0.0 or even the older 0.10.0 but 0.20.0 meant something is quite wrong. Found that we had a typo in the docs/conf.py

Testing

make install
make html
open build/html/2.0/index.html

to see the new expected output.

Links

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