Replace non-ASCII apostrophe (DataDog#392)

In some environments, a non-ASCII character causes a failure when pip installing datadog.
I could not replicate on my local OSX machine, but I have a Docker image based on Alpine Linux that fails reliably with the character in the README.

Author: Alphadash

README.md

@@ -95,7 +95,7 @@ Thread Safety
 Origin detection over UDP
 -------------
 Origin detection is a method to detect which pod `DogStatsD` packets are coming from in order to add the pod's tags to the tag list.
-The `DogStatsD` client attaches an internal tag, `entity_id`. The value of this tag is the content of the `DD_ENTITY_ID` environment variable if found, which is the pod’s UID.
+The `DogStatsD` client attaches an internal tag, `entity_id`. The value of this tag is the content of the `DD_ENTITY_ID` environment variable if found, which is the pod's UID.
 This tag will be used by the Datadog Agent to insert container tags to the metrics. You should only `append` to the `constant_tags` list to avoid overwriting this global tag.
 
 To enable origin detection over UDP, add the following lines to your application manifest