[Docs] Fix typo in GenericsManifesto.md.

Minor clarification to "opening existentials" example.

Author: dan-zheng

docs/GenericsManifesto.md

@@ -757,8 +757,8 @@ if e1 == e2 { ... } // error: e1 and e2 don't necessarily have the same dynamic
 One explicit way to allow such operations in a type-safe manner is to introduce an "open existential" operation of some sort, which extracts and gives a name to the dynamic type stored inside an existential. For example:
 
 ```Swift
-if let storedInE1 = e1 openas T {     // T is a the type of storedInE1, a copy of the value stored in e1
-  if let storedInE2 = e2 as? T {      // is e2 also a T?
+if let storedInE1 = e1 openas T { // T is the type of storedInE1, a copy of the value stored in e1
+  if let storedInE2 = e2 as? T {  // does e2 have type T? if so, copy its value to storedInE2
     if storedInE1 == storedInE2 { ... } // okay: storedInT1 and storedInE2 are both of type T, which we know is Equatable
   }
 }