Be consistent with spelling of "S-expression"

[toddjonker]

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          Traditional convention is "S-expression" with upper S, lower e.

• https://amazon-ion.github.io/ion-docs/docs/spec.html#sexp
• https://en.wikipedia.org/wiki/S-expression

Originally posted by @toddjonker in https://github.com/amazon-ion/ion-docs/pull/219#discussion_r1197149405