message: Consistently use `--upgrade` instead of `-U`
diff: @@ -65,7 +65,7 @@ We *recommend* using `pip` version 8.1 or higher. If using a release before
  version 8.1, upgrade `pip`:
  <pre class="prettyprint lang-bsh">
  - <code class="devsite-terminal">pip install -U pip</code>
  + <code class="devsite-terminal">pip install --upgrade pip</code>
  </pre>
  If not using Ubuntu and [setuptools](https://pypi.org/project/setuptools/) is
  @@ -102,7 +102,7 @@ When the Virtualenv is activated, the shell prompt displays as `(venv) $`.
  Within the active virtual environment, upgrade `pip`:
  <pre class="prettyprint lang-bsh">
  -(venv)$ pip install -U pip
  +(venv)$ pip install --upgrade pip
  </pre>
  You can install other Python packages within the virtual environment without
  @@ -120,7 +120,7 @@ Choose one of the available TensorFlow packages for installation:
  Within an active Virtualenv environment, use `pip` to install the package:
  <pre class="prettyprint lang-bsh">
  - <code class="devsite-terminal">pip install -U tensorflow</code>
  + <code class="devsite-terminal">pip install --upgrade tensorflow</code>
  </pre>
  Use `pip list` to show the packages installed in the virtual environment.
  @@ -198,7 +198,7 @@ We *recommend* using `pip` version 8.1 or higher. If using a release before
  version 8.1, upgrade `pip`:
  <pre class="prettyprint lang-bsh">
  - <code class="devsite-terminal">pip install -U pip</code>
  + <code class="devsite-terminal">pip install --upgrade pip</code>
  </pre>
  If not using Ubuntu and [setuptools](https://pypi.org/project/setuptools/) is
  @@ -220,8 +220,8 @@ Choose one of the available TensorFlow packages for installation:
  And use `pip` to install the package for Python 2 or 3:
  <pre class="prettyprint lang-bsh">
  - <code class="devsite-terminal">pip install -U --user tensorflow # Python 2.7</code>
  - <code class="devsite-terminal">pip3 install -U --user tensorflow # Python 3.n</code>
  + <code class="devsite-terminal">pip install --upgrade --user tensorflow # Python 2.7</code>
  + <code class="devsite-terminal">pip3 install -upgrade --user tensorflow # Python 3.n</code>
  </pre>
  Use `pip list` to show the packages installed on the system.
  