Incorrect errList type in Handling Errors section of client_config.md documentation

[oka4shi]

Describe the bug
The type of errList is incorrect in the example code in the Handling errors section of docs/client_config.md.

Currently, errList is declared as *gqlerror.List, but errors.As doesn't correctly evaluate this type.
Additionally, the for statement in line 8 got the following error:
cannot range over errList(variable of type *gqlerror.List)

Expected behavior
The correct type of errList should be gqlerror.List, so the code will be like this:

// return both error and field:
resp, err := getUser(...)
return resp.User.Name, err

// handle different errors differently:
resp, err := getUser(...)
- var errList *gqlerror.List
+ var errList gqlerror.List
if errors.As(err, &errList) {
  for _, err := range errList {
    fmt.Printf("%v at %v\n", err.Message, err.Path)
  }
  fmt.Printf("partial response: %v\n", resp)
} else if err != nil {
  fmt.Printf("http/network error: %v\n", err)
} else {
  fmt.Printf("successful response: %v\n", resp)
}

genqlient version
v0.7.0

[benjaminjkraft]

Thanks for the report! See #369.