Minor typo: missing an s

Prompt for Activity 1.5.16f should read "Which of the following equations...."
[Activity 1.5.16](https://tbil.org/preview/precalculus/instructor/EQ5.html#complete-the-square-a-not-one) 
https://tbil.org/preview/precalculus/instructor/EQ5.html#complete-the-square-a-not-one