http: print correct HTTP string in verbose output when using HTTP/2

Before:
```
 % src/curl https://sigsegv.ninja/ -v --http2
...
> GET / HTTP/1.1
> Host: sigsegv.ninja
> User-Agent: curl/7.52.2-DEV
> Accept: */*
>
...
```

After:
```
 % src/curl https://sigsegv.ninja/ -v --http2
...
> GET / HTTP/2
> Host: sigsegv.ninja
> User-Agent: curl/7.52.2-DEV
> Accept: */*
>
```