Use reference-style links in Upgrade Guide

Author: jmikola

docs/upgrade-guide.md

@@ -83,7 +83,9 @@ between [updating specific fields][update] and
 [MongoDB\Collection][collection] does not yet have a helper method for the
 [group][group] command; however, that is planned in [PHPLIB-177][jira-group].
 The following example demonstrates how to execute a group command using
-[Database::command()](classes/database.md#command):
+[Database::command()][command]:
+
+[command]: classes/database.md#command
 
 ```php
 <?php
@@ -108,7 +110,7 @@ $resultDocument = $cursor->toArray()[0];
 [MongoDB\Collection][collection] does not yet have a helper method for the
 [mapReduce][mapReduce] command; however, that is planned in
 [PHPLIB-53][jira-mapreduce]. The following example demonstrates how to execute a
-mapReduce command using [Database::command()](classes/database.md#command):
+mapReduce command using [Database::command()][command]:
 
 ```php
 <?php
@@ -139,8 +141,11 @@ with [DBRef][dbref] objects; however, that is planned in
 
 [MongoCollection::save()][save], which was syntactic sugar for an insert or
 upsert operation, has been removed in favor of explicitly using
-[insertOne()](classes/collection.md#insertone) or
-[replaceOne()](classes/collection.md#replaceone) (with the "upsert" option).
+[insertOne()][insertone] or [replaceOne()][replaceone] (with the "upsert"
+option).
+
+[insertone]: classes/collection.md#insertone
+[replaceone]: classes/collection.md#replaceone
 
 ![save() flowchart](img/save-flowchart.png)
 
@@ -155,8 +160,9 @@ dangerous [full-document replacements][replace].
 ### MongoWriteBatch
 
 The legacy driver's [MongoWriteBatch][batch] classes have been replaced with a
-general-purpose [bulkWrite()](classes/collection.md#bulkwrite) method. Whereas
-the legacy driver only allowed bulk operations of the same time, the new method
-allows operations to be mixed (e.g. inserts, updates, and deletes).
+general-purpose [bulkWrite()][bulkwrite] method. Whereas the legacy driver only
+allowed bulk operations of the same time, the new method allows operations to be
+mixed (e.g. inserts, updates, and deletes).
 
 [batch]: http://php.net/manual/en/class.mongowritebatch.php
+[bulkwrite]: classes/collection.md#bulkwrite