Update content/collective_intro.tex

content/collective_intro.tex

@@ -19,7 +19,7 @@
 \end{enumerate}
 
 Concurrent accesses to symmetric memory by an \openshmem collective
-routine and any other means of access---where at least one PE updates the
+routine and any other means of access---where at least one \ac{PE} updates the
 symmetric memory---results in undefined behavior.
 Since \acp{PE} can enter and exit collectives at different times,
 accessing such memory remotely may require additional synchronization.