Clarify explanation of rolling updates

I was tripped up while reading the paragraph that explains what a rolling update is and how it works. This commit makes some small phrasing and grammatical changes to make the explanation easier to read. 

Importantly, it replaces the word "updates" with "replaces", since rolling updates create *new* pods which replace the old ones, rather than updating the current pods.

Author: john-gaughan

content/en/docs/tutorials/kubernetes-basics/update/update-intro.html

@@ -27,7 +27,7 @@ <h3>Objectives</h3>
             <div class="col-md-8">
             <h3>Updating an application</h3>
 
-            <p>Users expect applications to be available all the time and developers are expected to deploy new versions of them several times a day. In Kubernetes this is done with rolling updates. <b>Rolling updates</b> allow Deployments' update to take place with zero downtime by incrementally updating Pods instances with new ones. The new Pods will be scheduled on Nodes with available resources.</p>
+            <p>Users expect applications to be available all the time, and developers are expected to deploy new versions of them several times a day. In Kubernetes this is done with rolling updates. A <b>rolling update</b> allows a Deployment update to take place with zero downtime. It does this by incrementally replacing the current Pods with new ones. The new Pods are scheduled on Nodes with available resources.</p>
 
             <p>In the previous module we scaled our application to run multiple instances. This is a requirement for performing updates without affecting application availability. By default, the maximum number of Pods that can be unavailable during the update and the maximum number of new Pods that can be created, is one. Both options can be configured to either numbers or percentages (of Pods).
             In Kubernetes, updates are versioned and any Deployment update can be reverted to a previous (stable) version.</p>