[syncstream.syncbuf.cons] Remove bogus rdbuf() calls

You don't call rdbuf() to get to the streambuf, this type is the streambuf.

Author: jwakely

source/iostreams.tex

@@ -10975,7 +10975,7 @@
 Output stored in \tcode{other}
 prior to calling this constructor
 will be stored in \tcode{*this} afterwards.
-\tcode{other.rdbuf()->pbase() == other.rdbuf()->pptr()}
+\tcode{other.pbase() == other.pptr()}
 and
 \tcode{other.get_wrapped() == nullptr}
 are \tcode{true}.