Correct a printf format string reflecting 'unsigned int'

[eddelbuettel]

Closes #1447

Corrects a format string for the unsigned int variable used

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Preferably, new tests were added which fail without the change
• Document the changes by file in ChangeLog

[kevinushey]

LGTM!

[Enchufa2]

There is a more general problem in that file, because the private attribute is an unsigned int, but then it is used elsewhere as an int, which will probably bite us in the future.

[eddelbuettel]

Agree that it is a smell. I was probably too influenced by the POSIX data structure where I may have seen an unsigned int. It should not matter as we are 'range safe' here, the value (of fractional microseconds) cannot overflow an integer. Which also means it never needed to be an unsigned. Sigh. We could change it.