AlphaVantage: Wrong suffix for Stockholm

[larslj]

GnuCash cannot get data from the Stockholm stock exchange. The currency is missing since the suffix is incorrect in AlphaVantage.pm. The correct suffix is '.STO', e.g. ERIC-B.STO for Ericsson.

[bpschuck]

Verified issue:
`bschuck@valhalla:~$ gnc-fq-dump alphavantage ERIC-B.STO
Finance::Quote fields Gnucash uses:
symbol: ERIC-B.STO <=== required
date: 04/12/2019 <=== recommended
currency: ** missing ** <=== required
last: 91.4000 <=\
nav: <=== one of these
price: <=/
timezone: <=== optional

** This stock quote cannot be used by GnuCash!`

Can be easily fixed by a quick modification to the AlphaVantage module:

bschuck@valhalla:~$ gnc-fq-dump alphavantage ERIC-B.STO Finance::Quote fields Gnucash uses: symbol: ERIC-B.STO <=== required date: 04/12/2019 <=== recommended currency: SEK <=== required last: 91.4000 <=\ nav: <=== one of these price: <=/ timezone: <=== optional