Update introduction.md

[bryansoto]

Minor typo and inconsistent use of spelling of behavior/behaviour.

The use of behavior is on line 25 so luckily it will actually show up in the diff. I just changed line 26 to match it.

If behaviour is preferred, I'm happy to change it.

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[andrerfcsantos]

Thanks for this!

There were other places where these changes needed to be made. In the interest of not bothering you with more work, I updated the PR to also include the changes in the other files they were needed.