custom-titlepage: change color from "ffffff" to "FFFFFF"

to should avoid LaTeX complaining about a wrong color:

pandoc custom-titlepage/custom-titlepage.md -o custom-titlepage/custom-titlepage.pdf --from markdown --template ../eisvogel.latex --listings
Error producing PDF.
! Missing number, treated as zero.
<to be read again>
                   f
l.339 \color[HTML]{ffffff}

examples/custom-titlepage/custom-titlepage.md

@@ -7,8 +7,8 @@ keywords: [Markdown, Example]
 subtitle: "Aesculeae domus vincemur et Veneris adsuetus lapsum"
 titlepage: true
 titlepage-color: "06386e"
-titlepage-text-color: "ffffff"
-titlepage-rule-color: "ffffff"
+titlepage-text-color: "FFFFFF"
+titlepage-rule-color: "FFFFFF"
 titlepage-rule-height: 1
 ...