Changed the construction of divider string for the ImGuiShaderUtils.

[lemonade-dm]

It was using curly braces, now it is using parenthesis.
It was causing a compilation on a developers machine.

D:/github/AtomSampleViewer/Gem/Code/Source/Utils/ImGuiShaderUtils.cpp(32,46): error C2398: Element '1': conversion from 'unsigned __int64' to '_Elem' requires a narrowing conversion
3>        with
3>        [
3>            _Elem=char
3>        ]
3>            AZStd::string divider{header.size(), '-'};
3>                                             ^
3>D:/github/AtomSampleViewer/Gem/Code/Source/Utils/ImGuiShaderUtils.cpp(32,53): warning C4267: 'initializing': conversion from 'size_t' to '_Elem', possible loss of data
3>        with
3>        [
3>            _Elem=char
3>        ]
3>            AZStd::string divider{header.size(), '-'};

Signed-off-by: lumberyard-employee-dm 56135373+lumberyard-employee-dm@users.noreply.github.com