Change Pull CDN wording for clarity (donnemartin#55)

Author: bengadbois

README.md

@@ -589,7 +589,7 @@ Sites with a small amount of traffic or sites with content that isn't often upda
 
 ### Pull CDNs
 
-Pull CDNs grab new content from your server when the first user requests the content.  You leave the content on your server and rewrite URLs to point to the CDN.  This results in a slower request until the content is cached on the server.
+Pull CDNs grab new content from your server when the first user requests the content.  You leave the content on your server and rewrite URLs to point to the CDN.  This results in a slower request until the content is cached on the CDN.
 
 A [time-to-live (TTL)](https://en.wikipedia.org/wiki/Time_to_live) determines how long content is cached.  Pull CDNs minimize storage space on the CDN, but can create redundant traffic if files expire and are pulled before they have actually changed.