[tab:atomic.types.pointer.comp] Fix column captions. (#4137)

Author: Eelis

source/atomics.tex

@@ -2574,12 +2574,12 @@
 \begin{floattable}
 {Atomic pointer computations}{atomic.types.pointer.comp}{lll|lll}
 \hline
-\tcode{Key}       &
-  Op          &
-  Computation     &
-\tcode{Key}       &
-  Op          &
-  Computation     \\ \hline
+\hdstyle{\tcode{\placeholder{key}}}  &
+  \hdstyle{Op}                       &
+  \hdstyle{Computation}              &
+\hdstyle{\tcode{\placeholder{key}}}  &
+  \hdstyle{Op}                       &
+  \hdstyle{Computation}  \\ \hline
 \tcode{add}       &
   \tcode{+}       &
   addition        &