Grammatical errors fixed.

In the Callbacks section, there are grammatical errors, in the last paragraph, which is just below the code snippet. The sentence is written as, "Resolving a promise means that you either fulfill a promise with a value or reject a promise with a reason. Resolving a promises triggers callbacks registered with the promises's then method."

The corrected sentence could be written as, "Resolving a promise means that you either fulfill a promise with a value or reject a promise with a reason. Resolving a promise triggers callbacks registered with the promises' then method.

Thank you.