[streambuf.virt.put] clarify boolean expression

Author: Eisenwave

source/iostreams.tex

@@ -4073,7 +4073,7 @@
 and
 \tcode{pptr()}
 are set so that:
-\tcode{pptr() - pbase() == r}
+\tcode{pptr() - pbase() == r} is \keyword{true}
 and the \tcode{r} characters starting at
 \tcode{pbase()}
 are the associated output stream.