S3 diagnostics suggests correction with typo

[sven-stodtmann]

When missing @exportS3Method, I get the following, potentially very helpful message: S3 method method.cls needs @export or @exportS3method tag. However, when copy-pasting the suggestion, it leads to an error since "method" must be capitalized. It should be S3 method method.cls needs @export or @exportS3Method tag. instead.