[atomics.ref.float] Fix spelling of placeholder

Author: timsong-cpp

source/threads.tex

@@ -3725,7 +3725,7 @@
 There are specializations of the \tcode{atomic_ref} class template
 for all floating-point types.
 For each such type \tcode{\placeholder{floating-point-type}},
-the specialization \tcode{atomic_ref<\placeholder{floating-\-point}>} provides
+the specialization \tcode{atomic_ref<\placeholder{floating-\-point-type}>} provides
 additional atomic operations appropriate to floating-point types.
 
 \pnum