Change wording for Endpoint description

I was reading over the description of the Endpoint and it struck me as a bit odd:
`An Endpoint can belong to *only one* network but may only belong to *one* Sandbox.`
I just wanted to rephrase it so that it's clear that an Endpoint has a one to one relationship with the Sandbox and a Network. If that is not the case, then I'm sorry for proposing the change. I'm only just starting to take a deeper dive into Docker networking.

Signed-off-by: Sebastian Radloff <sradloff23@gmail.com>

libnetwork/docs/design.md

@@ -30,7 +30,7 @@ A Sandbox may contain *many* endpoints from *multiple* networks.
 
 An Endpoint joins a Sandbox to a Network.
 An implementation of an Endpoint could be a `veth` pair, an Open vSwitch internal port or similar.
-An Endpoint can belong to *only one* network but may only belong to *one* Sandbox.
+An Endpoint can belong to *only one* network and *one* Sandbox.
 
 **Network**