Update README.md, app.json (button fix + env value fix)

[Tjeker61]

1. README.md: One-click install button fix. Now the button works as it should.
2. app.json: "DB_POSTGRESDB_SSL_REJECT_UNAUTHORIZED" env value fixed. The default value there is now "false", not an empty field

[Tjeker61]

Knock, knock, is anybody home? @Joffcom

[arnaudhchrt123]

I can confirm that @Tjeker61's commit works, and the actual one isn't!

Thanks @Tjeker61, for saving me a headache!

[igillis-clara]

Thanks @Tjeker61 ! This worked for me