Fix a typo (#11009)

Need to change 'M' to 'm' otherwise it won't work. You may refer to https://github.com/neoclide/coc-prettier/pull/47

docs/vim.md

@@ -116,7 +116,7 @@ Update your `coc-settings.json` for languages that you want format on save.
 
 ```json
 {
-  "coc.preferences.formatOnSaveFiletypes": ["css", "Markdown"]
+  "coc.preferences.formatOnSaveFiletypes": ["css", "markdown"]
 }
 ```