Fix for a duplicate nullability suffix in Md renderers

[VladimirBrejcha]

It seems to me that this line:

buf.write(elementType.nullabilitySuffix);

adds nullabilitySuffix to the string, and then this line:

wrapNullability(elementType, buf.toString());

which leads to:

String wrapNullability(T elementType, String inner) =>
      '$inner${elementType.nullabilitySuffix}';

that does exactly the same, which results in duplicate nullability suffix in the resulting string

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[VladimirBrejcha]

@googlebot I signed it!

[jcollins-g]

Thanks for your contribution, and yep, this looks like a bug! Will approve/merge once all the checks go green.

[jcollins-g]

@VladimirBrejcha It looks like this PR may need an update from head to pass; some changes in the dev SDK required some commits to dartdoc.

[VladimirBrejcha]

@jcollins-g I merged latest changes from master

[coveralls]

Coverage increased (+0.001%) to 57.572% when pulling 0447df5 on VladimirBrejcha:master into 3d990ca on dart-lang:master.