Style improvements on MongoDB example (vercel#20515)

I've looked at the example code and saw some consistent issues related to code style. The changes applied to this PR fixes the following points:

- Differences of line breaks styles between multiple files
- Differences of if statements styles
- Unnecessary comment
- A typo on a JSDocs

---

There were line breaks between statements on `pages/index.js`
````
export async function getServerSideProps(context) {
  const { client } = await connectToDatabase()

  const isConnected = await client.isConnected() 

  return {
    props: { isConnected },
  }
}
```` 

And this wasn't being applied to the MongoDB utility:

````
export async function connectToDatabase() {
  if (cached.conn) return cached.conn
  if (!cached.promise) {
    const conn = {}
    const opts = {
      useNewUrlParser: true,
      useUnifiedTopology: true,
    }
{...}
````

And also, as shown in the snippet above, there are different styles of if statements being used. 

With that being said, the reason I made this PR is because I think that this kind of inconsistent arises questions when a contributor looks to the codebase, even if this is a simple example.

examples/with-mongodb/pages/index.js

@@ -225,7 +225,7 @@ export default function Home({ isConnected }) {
 export async function getServerSideProps(context) {
   const { client } = await connectToDatabase()
 
-  const isConnected = await client.isConnected() // Returns true or false
+  const isConnected = await client.isConnected()
 
   return {
     props: { isConnected },

examples/with-mongodb/util/mongodb.js

@@ -16,30 +16,42 @@ if (!MONGODB_DB) {
 
 /**
  * Global is used here to maintain a cached connection across hot reloads
- * in development. This prevents connections growing exponentiatlly
+ * in development. This prevents connections growing exponentially
  * during API Route usage.
  */
 let cached = global.mongo
-if (!cached) cached = global.mongo = {}
+
+if (!cached) {
+  cached = global.mongo = {}
+}
 
 export async function connectToDatabase() {
-  if (cached.conn) return cached.conn
+  if (cached.conn) {
+    return cached.conn
+  }
+
   if (!cached.promise) {
     const conn = {}
+
     const opts = {
       useNewUrlParser: true,
       useUnifiedTopology: true,
     }
+
     cached.promise = MongoClient.connect(MONGODB_URI, opts)
       .then((client) => {
         conn.client = client
+
         return client.db(MONGODB_DB)
       })
       .then((db) => {
         conn.db = db
+
         cached.conn = conn
       })
   }
+
   await cached.promise
+
   return cached.conn
 }