I've got a copy right here and (1) I can't find that footnote, or anything close enough that it might be a benignly garbled copy, in it (either in the main text or in footnotes) but (2) there's plenty very near the start that's like it in tone and that the footnote might well be a parody of. For instance, here's some material from near the start of the preface, some phrases of which you will recognize:

[...] as far as revision is concerned, there is something to be said for preserving a work in its original form, warts and all. I have thus followed a very conservative policy of correction for the present printing. [...] A good indication of my conservative policy is in footnote 56. In that footnote the letter-nomenclature for the various objects involved, inexplicably garbled in the original printing, has been corrected; but I make no mention of the fact that the argument of the footnote now seems to me to have problems which I did not know when I wrote it and which at least require further discussion.

To which he adds a footnote:

Although I have not had time for careful study of Nathan Salmon's criticism [...] of this footnote, it seems likely that his criticism of the argument, though related to mine, is not the same and reconstructs it in a way that does not correspond to my exact intent and makes the argument unnecessarily weak. [...]