docs: fix formatting button example

[leno23]

Summary

• Correct the README button example from format to formatting.

Fixes #469

Test plan

• static validation: "format" appeared 1 time(s) in README.md before replacement.
• static validation: "formatting" is present in README.md after patch.
• static validation: "format" is absent from README.md after patch.
• Not run: runtime test suite, because this is a small README/documentation typo fix.