Use "some_function" instead of "sum_fun" in warnings

[eksperimental]

Elixir v1.19 introduces a warning related to structs,

 hint: given pattern matching is enough to catch typing errors, you may optionally convert the struct update into a map update. For example, instead of:

     user = some_fun()
     %User{user | name: "John Doe"}

 it is enough to write:

     %User{} = user = some_fun()
     %{user | name: "John Doe"}

Since this could be seen by new-comers to the language, offering a better user experience by avoiding abbreviations. Favoring the usage of "some_function" instead of "some_fun"

[josevalim]

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