给定一段英文文本，将其中的所有逗号替换为分号，并返回修改后的文本。
文本：Natural language processing is a field of computer science, and it focuses on how to develop algorithms and technologies to enable computers to understand and generate human language.
Modified text: Natural language processing is a field of computer science; and it focuses on how to develop algorithms and technologies to enable computers to understand and generate human language.