error pattern match map ^key (key with pin)

in the manual

```
field = "founded"
%{^field: city_founded} = nola # city_founded now equals 1718
```

correct answer 
```
nola = %{ name: "New Orleans", founded: 1718 }
field = :founded
%{^field => city_founded} = nola # pay atention to this line
```