libstdc++: Fix comments in <sstream> tests that mention basic_filebuf

libstdc++: Fix comments in <sstream> tests that mention basic_filebuf

libstdc++-v3/ChangeLog:

	* testsuite/27_io/basic_istringstream/cons/2020.cc: Fix comment
	referring to basic_filebuf.
	* testsuite/27_io/basic_istringstream/requirements/base_classes.cc:
	Likewise.
	* testsuite/27_io/basic_ostringstream/cons/2020.cc: Likewise.
	* testsuite/27_io/basic_ostringstream/requirements/base_classes.cc:
	Likewise.
	* testsuite/27_io/basic_stringbuf/cons/2020.cc: Likewise.
	* testsuite/27_io/basic_stringbuf/requirements/explicit_instantiation/2.cc:
	Likewise.
	* testsuite/27_io/basic_stringbuf/requirements/explicit_instantiation/4.cc:
	Likewise.
	* testsuite/27_io/basic_stringstream/cons/2020.cc: Likewise.
	* testsuite/27_io/basic_stringstream/requirements/base_classes.cc:
	Likewise.