As far as I know there is no English translation for this story. Here are some references in Chinese:
https://special.chaoxing.com/special/screen/tocard/89816928?courseId=89816829
https://www.buddha.twmail.cc/2-2/read_compensation.htm
https://www.dharanipitaka.com/MCollection/Miscellaneous.pdf
http://www.guanzizai.com/music/804.html

The story goes that there were once two friends named Daoru (道如) and Linju (邻居). Daoru recited the Lotus Sutra ten thousand times. Sometime after Daoru completed that practice, his friend, Linju, died.

Since his friend had died, Daoru now did “Jijang Bosal” chanting. As he traveled through the Bardo, Linju could hear the chanting and it gave him encouragement. Jijang Bosal also heard the chanting. Soon, Linju found himself facing Jijang Bosal, who began to speak to Linju: “Your friend Daoru is chanting for you now, which is very good pracice. But Daoru always makes many mistakes when he chants. When he chanted the Lotus Sutra ten thousand times, he made one hundred mistakes each time he chanted the Sutra. So he made one million mistakes!”

Then Jijang Bosal told Linju that instead of leading Linju on to his next life, he was going to teach Linju a mantra and then send him back to his previous life so he could teach the mantra to Daoru and everyone else. Everyone should chant this mantra in order to correct the mistakes we make in our chanting. After being “dead” for seven days, Linju suddenly recovered and told everyone about his encounter with Jijang Bosal, and the mantra he had been taught.

解說：此真言又稱補缺真言。全名為「法華補闕真言」或「地藏菩薩補闕真言」，為夢授咒。

此真言是蜀郡靜泉寺僧道如，俗姓李，發願讀《法華經》一萬遍。至元和初遍數滿。鄰房有一僧，死經七日卻活來云：見地藏菩薩，有冥司與卒等俱時至，問僧云：汝與道如鄰房住否？答曰：然也。地藏菩薩云：道如持經一萬遍滿畢，然以漏闕至多，冥官藏不能納之。汝持此明與彼道如，每誦經一遍即誦此「明」（指此補缺真言）三遍，所遺闕文句即得圓滿如法。冥官攝受云云。

此真言 = this mantra
又稱 = also known as
補缺 = fill a vacancy
真言 = mantra
。
全名為 = full name for (something)
法華補闕真言 = Dharma Brilliant Correct Mistakes Mantra
或 = or
地藏菩薩補闕真言 = Dizang Bodhisattva’s Correct Mistakes Mantra
，
為 = for
夢 = dream
授 = give
咒 = mantra
。
此真言是 = this mantra is
蜀郡 = Shu county
靜泉寺 = jìng quán temple (silent spring temple)
僧 = monk
道如 = Daoru
，
俗姓李 = whose common name is Li
，
發願 = make a vow
讀 = say aloud, recite
《法華經》 = Lotus Brilliant Sutra
一萬遍 = ten thousand times
。

至元
和初遍數滿
。
鄰房有一僧
，

補 闕 眞言
보 궐 진언
bo gwol jinon

唵 好魯好魯 舍野謨契 娑婆訶
옴 호로호로 사야모케 사바하
om hororo sayamoke sabaha