This is a logos because it is a fact about America history which then helps support what he says next.
"Five score years ago, a great American, in whose symbolic shadow we stand today, signed the Emancipation Proclamation."Pg. 195
"I have a dream that my four little children will one day live in a nation where they will not be judged by the color of their skin, but by the content of their character."Pg.199
When Dr. King talks about his kids and how he wants a better life for them, a lot of people could relate with him which also helped them sympathize with him.
"I am happy to join with you today in what will go down in history as the greatest demonstration for freedom in the history of our nation."Pg.195
When he starts his speech he implies that everyone is together by using words such as "join" and "our" which subconsciously creates trust with the audience. He also talks about the U.S. which shows that he has interest in the U.S. and its well being.