Minor Spelling issues

Hi Oliver,

While updating translations I came across two spelling issues.

1. dissmissable > dismissable in Readme
Permalink – https://translate.ww.wp.xz.cn/projects/wp-plugins/options-for-twenty-seventeen/dev-readme/en-ca/default?filters%5Bstatus%5D=either&filters%5Boriginal_id%5D=7142291&filters%5Btranslation_id%5D=60042932

2. Enabe > Enable In options-for-twenty-seventeen.php
Permalink – https://translate.ww.wp.xz.cn/projects/wp-plugins/options-for-twenty-seventeen/dev/en-ca/default?filters%5Bstatus%5D=either&filters%5Boriginal_id%5D=7025861&filters%5Btranslation_id%5D=60042921

All the best,
Cheers