New erratum (#29) for R7RS-small

[John Cowan]

Andy WIngo pointed out on #guile that the definition of `symbol=?` has the same problem as the definition of `boolean=?`, namely that it says to return false if any of the arguments is not a symbol.  In fact, it's an error to pass anything but a symbol.

I've updated the errata list at <https://bitbucket.org/cowan/r7rs-wg1-infra/src/default/R7RSSmallErrata.md> and the TeX source and PDF at <https://bitbucket.org/cowan/r7rs/src/errata>.  I've also filed a Chibi issue for both predicates, although Chibi technically conforms to original and corrected versions.