。
He added a comma in a separate paragraph.
Literal
Different paragraph [in-で] he [topic-は] comma [object-を] one inserted.
段落 is 'paragraph.' コンマ is the English loanword 'comma.' １つ is the native counter acting as an adverb: 'one (comma).' 入れる means 'to insert/put in.' A sentence about editing — straightforward but specific to writing contexts.