Add a missing variable 't' in the integral of Example 11.2.2

The derivative of position is -9.8t + 15. The integration should be
done on the same function.
Fix a typo "definite integral" -> "indefinite integral" in the chapter about antiderivative
Merge pull request #89 from BenjaminPoulain/typo-11.1.6

Fix a typo "definite integral" -> "indefinite integral" in the chapter a...
Merge pull request #88 from BenjaminPoulain/typo-physics-integral

Add a missing variable 't' in the integral of Example 11.2.2
Merge branch 'development'
Merge branch 'development'
Merge branch 'development'