luci-app-ddns: Update global cacert path doc to reflect it is a file not a dir

[kontaxis]

Description:

1. The UI (Dynamic DNS > Global Settings) describes cacert as the path for certificates (plural)
2. However, code in packages/net/ddns-scripts/files/usr/bin/ddns.sh expects the parameter to be a file path, i.e., a certificate bundle, not a directory
3. Therefore (1) is misleading and if someone supplies a directory, e.g., /etc/ssl/certs, DDNS services list updates silently fail

I would expect the following to succeed given how this parameter is documented:

# uci get ddns.global.cacert
/etc/ssl/certs/
# ls /etc/ssl/certs/
ca-certificates.crt
# /usr/bin/ddns service update
Certification file not found (/etc/ssl/certs/)

When a file path is used instead, DDNS service update succeeds:

# uci get ddns.global.cacert
/etc/ssl/certs/ca-certificates.crt
# /usr/bin/ddns service update
Downloading 'https://raw.githubusercontent.com/openwrt/packages/master/net/ddns-scripts/files/usr/share/ddns/list'
[...]
Download completed (825 bytes)

This change updates the language in Dynamic DNS > Global Settings to indicate that cacert is a file and not a directory.

[systemcrash]

In this case, let's avoid ambiguity of path, and refer directly to the file itself. Please re-word to

		o = s.taboption('global', form.Value, 'cacert', _('CA Certs bundle file'));
		o.description = _('CA bundle file used to download services data. Set IGNORE to skip certificate validation.');