Fix Docker run command example in README

[michelesr]

When trying to launch the command:

docker -- run -p 10000:10000 gcr.io/android-battery-historian/stable:3.0 --port 10000
docker: 'run' is not a docker command.
See 'docker --help'

Removing -- fixes the problem:

docker run -p 10000:10000 gcr.io/android-battery-historian/stable:3.0 --port 10000                                                                                                
Unable to find image 'gcr.io/android-battery-historian/stable:3.0' locally
3.0: Pulling from android-battery-historian/stable
c62795f78da9: Pull complete
d4fceeeb758e: Pull complete
5c9125a401ae: Pull complete
0062f774e994: Pull complete
6b33fd031fac: Pull complete
a6bd6e1d0bdb: Pull complete
76cf9d0635af: Pull complete
856d20d533e0: Pull complete
e63a73f6a528: Pull complete
1a75578c9353: Pull complete
24f3649604d9: Pull complete
10f637765748: Pull complete
e06a9fa76cf2: Pull complete
Digest: sha256:265a37707f8cf25f2f85afe3dff31c760d44bb922f64bbc455a4589889d3fe91
Status: Downloaded newer image for gcr.io/android-battery-historian/stable:3.0
2019/03/15 09:36:53 Listening on port:  10000

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[michelesr]

I signed it!

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