Fixes typo in products.rst

an unique -> a unique

Details (see https://owl.english.purdue.edu/owl/resource/591/01/):

When "u" makes the same sound as the "y" in "you," or "o" makes the same sound as "w" in "won," then a is used. The word-initial "y" sound ("unicorn") is actually a glide [j] phonetically, which has consonantal properties; consequently, it is treated as a consonant, requiring "a."

- a union
- a united front
- a unicorn
- a used napkin
- a U.S. ship