docs: fix typos and grammar in contributor docs

[Xavrir]

Changes:

Fixes three small typos/grammar issues found while reading through the contributor documentation:

• documentation_style_guide.md (line 237): "Use describe() to in p5.js example code, to add" had an extra "to" — corrected to "Use describe() in p5.js example code to add"
• contributing_to_the_p5js_reference.md (line 1193): "This involves run the website locally" → "This involves running the website locally"
• project_wrapups/katiejliu_gsoc_2021.md (line 6): "specfically" → "specifically"

Screenshots of the change:

Not applicable — text-only corrections.

PR Checklist

• npm run lint passes
• Inline reference is included / updated
• Unit tests are included / updated

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