Added time unit (i.e. seconds) in `timeit.timeit()` in `intro_to_graph`

[harshdhamecha]

Added time unit (i.e. seconds) in timeit.timeit() in intro_to_graph to make it easily understand by users.

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[8bitmp3]

@harshdhamecha Since the bot isn't applied formatting automatically, here's what the CI error message says :

Format notebook: site/en/guide/intro_to_graphs.ipynb

[test] The following notebooks are not formatted:
- site/en/guide/intro_to_graphs.ipynb
Please install `nbfmt` and format:
$ python3 -m pip install -U --user git+https://github.com/tensorflow/docs
$ python3 -m tensorflow_docs.tools.nbfmt notebook.ipynb

So, if you can:

$ python3 -m pip install -U --user git+https://github.com/tensorflow/docs
$ python3 -m tensorflow_docs.tools.nbfmt intro_to_graph.ipynb

which should remove some vscode and other metadata.