Update example.rst

Created by: aliasgherman

1. The example code had missing n_clf variable definition. Added it as number of classifiers.
2. There was a mismatched parenthesis in the voting example. Removed it to make the example work
3. I have tried to copy the modified code into a fresh notebook and it works seamlessly
4. No tests or modules are added for this change
5. This should be considered as a documentation improvement only. (No code changes)

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