uconvert documentation update

Under quickstart in the documentation the uconvert function as written does not work. The documentation should be updated. The first line is a direct copy from the documentation and the second line is what works.

```julia
julia> uconvert(°C, 212°F)
ERROR: UndefVarError: °F not defined
Stacktrace:
[1] top-level scope
@ REPL[24]:1

julia> uconvert(u"°C", 212u"°F")
100//1 °C
```

Start in the documentation quickstart and locate the uconvert example shown in the issue. Update the example to use the working unit syntax, then verify that the rendered documentation and example no longer produce the reported UndefVarError.

Written by the indexing model from the issue text.