Fix a typo - #11009

Description

Need to change 'M' to 'm' otherwise it won't work. You may refer to neoclide/coc-prettier#47

Checklist

• [x ] I’ve added tests to confirm my change works.
• [ x] (If changing the API or CLI) I’ve documented the changes I’ve made (in the docs/ directory).
• [ x] (If the change is user-facing) I’ve added my changes to changelog_unreleased/*/XXXX.md file following changelog_unreleased/TEMPLATE.md.
• [ x] I’ve read the contributing guidelines.

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