Fix README: custom User-Agent token is prepended, not appended

The README incorrectly described the custom User-Agent token as being
appended to the default. The actual code in src/Dnsimple/Client.php
prepends it (customUserAgent first, then DEFAULT_USER_AGENT). This
commit corrects the description and the example output to match the
actual behavior.

Author: weppos

README.md

@@ -84,7 +84,7 @@ use Dnsimple\Client;
 $client = new Client("API_TOKEN", ["user_agent" => "my-app/1.0"]);
 ```
 
-The value you provide will be appended to the default `User-Agent` the client uses. For example, if you use `my-app/1.0`, the final header value will be `dnsimple-php/x.x.x my-app/1.0` (note that it will vary depending on the client version).
+The value you provide will be prepended to the default `User-Agent` the client uses. For example, if you use `my-app/1.0`, the final header value will be `my-app/1.0 dnsimple-php/x.x.x` (note that it will vary depending on the client version).
 
 ## Documentation