libstdc++: Fix Doxygen warning

This fixes a Doxygen warning about a mismatched parameter name. The
standard uses 'r' here, like the Doxygen comment, so use '__r' instead
of '__e'.

libstdc++-v3/ChangeLog:

	* include/bits/ptr_traits.h (pointer_traits::pointer_to): Rename
	parameter.
1 file changed