has-city: Specify type of paramaters/return values

Also some grammar fixes.

subjects/has-city/README.md

@@ -2,10 +2,10 @@
 
 ### Instructions
 
-Create a function `hasCity` that takes a country and an array of cities
-of that country and it returns a new function. \
-The new function takes a city parameter which returns
-whether a city is part of that country or not.
+Create a function `hasCity` that takes a string called `country` and a array of strings named `cities`,
+which contains the cities of that country and it returns a new function. \
+The new function takes a string called `city` as a parameter which returns
+whether the string is part of the `cities` array or not.
 
-If it is, it should be returned `<city> is a city from <country>`, otherwise
+If it is, it should return the string `<city> is a city from <country>`, otherwise
 `<city> is not a city from <country>`.