expression: add 2/(x+3) to acceptable formats

Summary:
Demonstrates how to enter polynomial denominators in fractions.
Prompted by issue reports such as AI-64780.

Test Plan:
 * make an expression in the answer area
 * look at the acceptable formats

http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%22%2C%22widgets%22%3A%7B%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20expression%201%5D%5D%22%2C%22widgets%22%3A%7B%22expression%201%22%3A%7B%22type%22%3A%22expression%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22value%22%3A%222%2F(x%2B3)%22%2C%22form%22%3Afalse%2C%22simplify%22%3Afalse%2C%22times%22%3Afalse%2C%22functions%22%3A%5B%22f%22%2C%22g%22%2C%22h%22%5D%7D%7D%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22hints%22%3A%5B%5D%7D

Auditors: eater