doc: fix explanation of package.json "type" field

Remove erroneous reference to files with `.mjs` extension, which are not
affected by the package.json "type" field.

doc/api/esm.md

@@ -59,9 +59,9 @@ or when referenced by `import` statements within ES module code:
 
 ### `package.json` `"type"` field
 
-Files ending with `.js` or `.mjs`, or lacking any extension,
-will be loaded as ES modules when the nearest parent `package.json` file
-contains a top-level field `"type"` with a value of `"module"`.
+Files ending with `.js` or lacking any extension will be loaded as ES modules
+when the nearest parent `package.json` file contains a top-level field `"type"`
+with a value of `"module"`.
 
 The nearest parent `package.json` is defined as the first `package.json` found
 when searching in the current folder, that folder’s parent, and so on up