[NUI] Resolve TextLable build warning

[NUI] Resolve TextLable build warning

Reference documents : https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/xmldoc/

To refer to XML elements (for example, your function processes specific XML elements that you want to describe in an XML documentation comment), you can use the standard quoting mechanism (< and >).

Signed-off-by: Eunki Hong <eunkiki.hong@samsung.com>