[Oops.] Misspelling of "Ackermann" as "Ackerman"

[echuber2]

Error is present in June 13 version.

Chapter number or note title: Section 7.5

Page number: p. 267

Error description: footnote mentions "inverse Ackerman" [sic]

Suggested fix (if any): I think it's spelled "Ackermann" (two n)

I just realized this version of the book doesn't include the longer notes on disjoint sets from the old course notes. Sad feels.