Update reactivity-core.md (vuejs#2068)

The wrong form of 'to be' verb is used here.

Author: joedeluca

src/api/reactivity-core.md

@@ -23,7 +23,7 @@ Takes an inner value and returns a reactive and mutable ref object, which has a
 
 - **Details**
 
-  The ref object is mutable - i.e. you can assign new values to `.value`. It is also reactive - i.e. any read operations to `.value` is tracked, and write operations will trigger associated effects.
+  The ref object is mutable - i.e. you can assign new values to `.value`. It is also reactive - i.e. any read operations to `.value` are tracked, and write operations will trigger associated effects.
 
   If an object is assigned as a ref's value, the object is made deeply reactive with [reactive()](#reactive). This also means if the object contains nested refs, they will be deeply unwrapped.