Fix README typo

Fixed a small typo/error with "the a" host

README.md

@@ -17,7 +17,7 @@ Then start any containers you want proxied with an env var `VIRTUAL_HOST=subdoma
 
 The containers being proxied must [expose](https://docs.docker.com/engine/reference/run/#expose-incoming-ports) the port to be proxied, either by using the `EXPOSE` directive in their `Dockerfile` or by using the `--expose` flag to `docker run` or `docker create`.
 
-Provided your DNS is setup to forward foo.bar.com to the a host running nginx-proxy, the request will be routed to a container with the VIRTUAL_HOST env var set.
+Provided your DNS is setup to forward foo.bar.com to the host running nginx-proxy, the request will be routed to a container with the VIRTUAL_HOST env var set.
 
 ### Image variants