[atomics.types.memop] Add missing \pnums (#1890)

Author: timsong-cpp

source/atomics.tex

@@ -1416,6 +1416,7 @@
 \end{itemdecl}
 
 \begin{itemdescr}
+\pnum
 \effects Equivalent to: \tcode{return fetch_add(1);}
 \end{itemdescr}
 
@@ -1427,6 +1428,7 @@
 \end{itemdecl}
 
 \begin{itemdescr}
+\pnum
 \effects Equivalent to: \tcode{return fetch_sub(1);}
 \end{itemdescr}