Update part6d.md

Minor grammar fix from "the solution have to be changed" to "the solution has to be changed".

Author: cabbachew

src/content/6/en/part6d.md

@@ -803,6 +803,6 @@ Over the years, there have also been other state management libraries developed
 
 Also, Redux does not have to be used in its entirety in an application. It may make sense, for example, to manage the form state outside of Redux, especially in situations where the state of a form does not affect the rest of the application. It is also perfectly possible to use Redux and React Query together in the same application.
 
-The question of which state management solution should be used is not at all straightforward. It is impossible to give a single correct answer. It is also likely that the selected state management solution may turn out to be suboptimal as the application grows to such an extent that the solution have to be changed even if the application has already been put into production use.
+The question of which state management solution should be used is not at all straightforward. It is impossible to give a single correct answer. It is also likely that the selected state management solution may turn out to be suboptimal as the application grows to such an extent that the solution has to be changed even if the application has already been put into production use.
 
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