Replace DirectiveGraphType with Directive

[anvu03]

Replace DirectiveGraphType with Directive in Directive.md since DirectiveGraphType has been renamed to Directive.

[sungam3r]

Thanks.

[micheltol]

@sungam3r The live-docs do not reflect this change yet. Am I missing something or does something else still need to be done? Can I do something to speed up the process?

[sungam3r]

@Shane32 https://github.com/graphql-dotnet/graphql-dotnet/runs/6837442454?check_suite_focus=true build is green but I still see DirectiveGraphType here https://graphql-dotnet.github.io/docs/getting-started/directives#repeatable-directives

[Shane32]

@micheltol The documentation has been fixed.