[streambuf.virt.pback] clarify boolean expression

Author: Eisenwave

source/iostreams.tex

@@ -3987,7 +3987,7 @@
 call this virtual function only when
 \tcode{gptr()}
 is null,
-\tcode{gptr() == eback()},
+\tcode{gptr() == eback()} is \keyword{true},
 or
 \tcode{traits::eq(traits::to_char_type(c), gptr()[-1])}
 returns