This phrase is a heading in a business-Japanese book. Underneath, the book talks about avoiding casual contracted words in the office and on phone calls.
From what I know about 留める(とどめる), which is a transitive verb, I would think 「...を留めましょう」 would be correct here. Although I can find examples of 「...に留める」, I can't find any that, to me, would make sense here. What does 「...に留める」 mean in this context?