Fixup (lib): name of "a standard library class" meta variable

Author: tkoeppe

source/lib-intro.tex

@@ -3103,10 +3103,10 @@
 \end{note}
 
 \pnum
-Let \tcode{\placeholder{F}} denote
+Let \tcode{\placeholder{C}} denote
 a standard library class or class template specialization.
 It is unspecified if or how
-a reflection value can be formed to any private member of \tcode{\placeholder{F}},
+a reflection value can be formed to any private member of \tcode{\placeholder{C}},
 or what the names of such members may be.
 
 \pnum