Fix typo

[Overlandcoder]

https://www.sqlteaching.com/#!substr

Small typo on Lesson 30, in this sentence:

"1 would indicate the first character, 2 would indicated the second character"

In that sentence, "2 would indicated" should be changed to "2 would indicate"

I realize this is minor, but I still wanted to contribute. And thanks for the lessons, they're great.