Docs: Wrong color value is displayed for secondary color in v4

Created by: abirana

This is really a minor issue, since we are already moving with v5 but haven't noticed till today. Similar color mismatch happen to our own templates/implementation as well, so I just wanted to post it.

In the Theme Color section, color value of secondary color is shown as #868e96 which is wrong. It should be #6c757d according to Bootstrap color values.