Docs: Fix single quote issue in postbox and media JS DocBlock

[dilipbheda]

Fix inline documentation.

[sabernhardt]

The apostrophe was in the correct spot for referring to multiple inputs (and their attributes/values). Am I missing sonething?

[SergeyBiryukov]

Thanks for the patch! The existing apostrophes are indeed correct, see ​Apostrophe Rules by Grammarly:

For most singular nouns, add apostrophe+s:
Example: The dog’s leash The writer’s desk The planet’s atmosphere

For most plural nouns, add only an apostrophe:
Example: The dogs’ leashes (multiple dogs) The writers’ desks (multiple writers) The planets’ atmospheres (multiple planets)