Mark The Words - Instructions on the content revisions

Make sure that the instructions on the content editor is consistent and keep it on singular form to so that users will know that only one word can be used as an answer not multiple words or phrases.

The following changes has been proposed in: https://h5p.org/comment/31202#comment-31202

"Correct words are marked with asterisks before and after the word." can be changed to "Correct word is marked with asterisks before and after the word."

"Asterisks can be added within marked words by adding another asterisk, correctword** => correctword*."

correctword may be replaced by 'answer' or 'word'