Some informal problems do not have the informal_solution field

[ArthurConmy]

This is the first problem that does not have the informal_solution field: https://github.com/trishullab/PutnamBench/blob/c092d1a/informal/putnam.json#L168

I think this is a bug, since many other problems have "None." for the informal_solution field. E.g. here: https://github.com/trishullab/PutnamBench/blob/c092d1a/informal/putnam.json#L160

[GeorgeTsoukalas]

Good catch! This was missed in a fairly high number (67) of problems. I have made a fix in #258. Thank you for pointing this out!