Netstandard2.0 doesn't support single char. Double quote strings requ…

…ired

src/Spectre.Console/Color.cs

@@ -220,7 +220,7 @@ public static Color FromInt32(int number)
     /// <returns>The color created from the hexadecimal string.</returns>
     public static Color FromHex(string hex)
     {
-        if (hex.StartsWith('#'))
+        if (hex.StartsWith("#"))
         {
             hex = hex.Substring(1);
         }