message: fix typo I added.
diff: @@ -221,7 +221,7 @@ And use `pip` to install the package for Python 2 or 3:
  <pre class="prettyprint lang-bsh">
  <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>
  + <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.
  