fix the default gateway port

reported by @sixcorners in
https://github.com/ipfs/go-ipfs/pull/5393#issuecomment-417908209

License: MIT
Signed-off-by: Steven Allen <steven@stebalien.com>

docs/gateway.md

@@ -4,7 +4,7 @@ An IPFS Gateway acts as a bridge between traditional web browsers and IPFS.
 Through the gateway, users can browse files and websites stored in IPFS as if
 they were stored in a traditional web server.
 
-By default, go-ipfs nodes run a gateway at `http://127.0.0.1:5001/`.
+By default, go-ipfs nodes run a gateway at `http://127.0.0.1:8080/`.
 
 We also provide a public gateway at `https://ipfs.io`. If you've ever seen a
 link in the form `https://ipfs.io/ipfs/Qm...`, that's being served from *our*