Update article.md

Correction of grammatical error

Author: Gleb-Pastushenko

1-js/08-prototypes/04-prototype-methods/article.md

@@ -116,7 +116,7 @@ alert(obj[key]); // [object Object], not "some value"!
 
 Here, if the user types in `__proto__`, the assignment in line 4 is ignored!
 
-That could surely be surprising for a non-developer, but pretty understandable for us. The `__proto__` property is special: it must be either an object or `null`. A string can not become a prototype. That's why an assignment a string to `__proto__` is ignored.
+That could surely be surprising for a non-developer, but pretty understandable for us. The `__proto__` property is special: it must be either an object or `null`. A string can not become a prototype. That's why assigning a string to `__proto__` is ignored.
 
 But we didn't *intend* to implement such behavior, right? We want to store key/value pairs, and the key named `"__proto__"` was not properly saved. So that's a bug!