Hola Paul,

the word "policía" in Spanish referring to "the police" (the police force) is feminine "la policía".

If we are referring to a policeman or policewoman then we say "el policía" or "la mujer policía". The feminine in singular is not commonly used. I guess this way we distinguish it from the "force", to avoid confusion. In plural, referring to some policemen or policewomen it would be "los policías" or "las policías". 

I will change the sentence in the lesson as it is in plural and should say in English "The policemen", not "the police".

I hope this clarified it.

Un saludo

Inma