Did I write this sentence right? Should there be an apostrophe in the word reader's?

Paragraph three is missing colorful vocabulary that would catch the reader’s attention.

Correct! The 's = the attention of the reader.

Sra

Yes, you wrote the sentence correctly. The word "reader's" in your sentence is in possessive form, indicating that something belongs to the reader. In this case, it refers to the attention of the reader. Adding an apostrophe before the "s" correctly shows possession in the word "reader's." So, the sentence is grammatically accurate.