message: Fix step numbering in Linux install docs
diff: @@ -151,10 +151,10 @@ Take the following steps to install TensorFlow with Virtualenv:
  (tensorflow)$ <b>pip install --upgrade tensorflow-gpu</b> # for Python 2.7 and GPU
  (tensorflow)$ <b>pip3 install --upgrade tensorflow-gpu</b> # for Python 3.n and GPU</pre>
  - If the preceding command succeeds, skip Step 5. If the preceding
  - command fails, perform Step 5.
  + If the preceding command succeeds, skip Step 6. If the preceding
  + command fails, perform Step 6.
  - 5. (Optional) If Step 4 failed (typically because you invoked a pip version
  + 6. (Optional) If Step 5 failed (typically because you invoked a pip version
  lower than 8.1), install TensorFlow in the active virtualenv environment
  by issuing a command of the following format:
  