Changed square to circle

[ibnlanre]

Changes proposed in this pull request:

• the "square" should have been a "circle" in conformity with its heading

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[chrome-devrel-infra[bot]]

:white_check_mark: Preview (static) for commit 18151445279dc2bf462e4666cf838de51ed7ecdd available at https://pr-6103-static-dot-dcc-staging.uc.r.appspot.com/.

The following pages likely changed with this PR:

• https://pr-6103-static-dot-dcc-staging.uc.r.appspot.com/articles/css-nesting/