docs: fix typo

[premdood]

Summary
The Document class is derived from Modal class

[vkarpov15]

What makes you think this is a typo? In Mongoose, Model inherits from Document:

mongoose/lib/model.js

Lines 116 to 128 in 0c5f56f

	function Model(doc, fields, skipId) {
	if (fields instanceof Schema) {
	throw new TypeError('2nd argument to `Model` constructor must be a POJO or string, ' +
	'**not** a schema. Make sure you\'re calling `mongoose.model()`, not ' +
	'`mongoose.Model()`.');
	}
	if (typeof doc === 'string') {
	throw new TypeError('First argument to `Model` constructor must be an object, ' +
	'**not** a string. Make sure you\'re calling `mongoose.model()`, not ' +
	'`mongoose.Model()`.');
	}
	Document.call(this, doc, fields, skipId);
	}

[premdood]

After looking into this further, I realised this isn't actually a typo. The documentation is correct as written. Model inherits from Document by calling the Document constructor.
Apologies for the confusion.