main/postmarketos-ui-phosh: fix desktop file name in mimeapps.list

firefox.desktop -> org.mozilla.firefox.desktop.

Related: https://git.alpinelinux.org/aports/commit/community/firefox?id=2d291a4afc2c71e25c8fd9fff188eff436b80580

Otherwise links are still not opened in Firefox, if its version is >=77.0.1-r4.