Answered by Shaykh Hamza Karamali, SunniPath Academy Teacher
Recite the du`a of qunut during Witr Prayer in the Second Half of Ramadan
It is recommended to make the standing supplication (Ar. du’a al-qunut) in the last rak’ah of witr after rising from ruku`, during the second half of Ramadan. This practice starts on the night of the sixteenth (i.e. the night before the sixteenth day) of Ramadan. If one forgets, it is recommended to prostrate out of forgetfulness.
The qunoot is a main sunna (or “sunna mu’akkada”) in the fajr prayer and in the witr during the second half of Ramadan. [Mostafa Elqabanny]
The specific du’a mentioned in the fiqh books (allahumma-hdina fi man hadayt wa ‘afina fi man ‘afayt etc.) is not required to fulfill the main sunna. One may confine oneself to any dhikr that comprises:
(1) a supplication (du’a)
(2) a praise of Allah ( thana ‘)
(3) invocation of blessings (salat) and peace (salam) on the Prophet (nabi), his folk (aal) and companions (sahb).
(al-Yaqut al-Nafis, p. 37)
The formula: “allaahumma-ghfir lee yaa ghafoor allahumma salli ‘ala sayyidina muhammad wa ‘alaa aalihi wa sahbihi wa sallim” meets the above requirements and is therefore sufficient. (Fath al-‘Allam, 2.251)
Note that the Reliance does not fully specify requirement (3) above. I have reproduced the relevant section from the Reliance below and added comments to clarify the intent of the text.
f8.53 (…) The words of this supplication are not set and (h: the main sunna of the qunut) may be accomplished by pronouncing any supplication (O: and praise) or Koranic verse containing a supplication, such as the last verses of al=Baqara (Koran 2:285-86), though the above words (h: meaning the famous du’a most often recited in qunut) are better. After this, one invokes blessings (h: and peace) on the Prophet (Allah bless him and give him peace) (h: and also on his folk and companions. Sending blessings and peace on the Prophet, his folk, and companions is also a main sunna whose nonperformance calls for the forgetfulness prostration).
If one omits (whether forgetfully or otherwise) the qunoot in the fajr prayer or the witr prayer in the second half of Ramadan, it is sunna to repair the damage with the forgetfulness prostration. [Hamza Karamali]
And Allah knows best.
 The formula means: “O Allah, O Oft-Forgiving One, forgive me! O Allah, send blessings and peace on our Master Muhammad, his folk and his companions”
Requirement (1) is fulfilled by “forgive me”,
Requirement (2) is fulfilled by “O Oft-Forgiving One”,
Requirement (3) is fulfilled by “O Allah, send blessings and peace on our Master Muhammad, his folk, and his companions”.