Fix typo in CAP theorem section (donnemartin#205)

Author: Varun

README.md

@@ -455,7 +455,7 @@ Waiting for a response from the partitioned node might result in a timeout error
 
 #### AP - availability and partition tolerance
 
-Responses return the most recent version of the data available on the a node, which might not be the latest.  Writes might take some time to propagate when the partition is resolved.
+Responses return the most recent version of the data available on a node, which might not be the latest.  Writes might take some time to propagate when the partition is resolved.
 
 AP is a good choice if the business needs allow for [eventual consistency](#eventual-consistency) or when the system needs to continue working despite external errors.