Typos on pages 42 and 43

[tracymedcalf]

Misspelled activation as "activiation" in paragraphs 2 and 3 of page 42.

In the 3rd paragraph of page 43, the text says "algorithm 4.2" twice when it should say "algorithm 5" and "algorithm 6".

[j-j-wright]

In the line of text immediately following equation 4.1, "it's" should instead be "its".