minor fix.

theory/paper_pasc/pasc_paper.tex

@@ -563,7 +563,7 @@ output to $800^3=5.12\times10^8$, $600^3=2.16\times10^8$ and
 $376^3=5.1\times10^7$ particles respectively. Scripts to generate these initial
 conditions are provided with the source code. We then run the \swift code for
 100 time-steps and average the wall clock time of these time-steps after having
-removed the first and last ones, where disk i/o occurs.
+removed the first and last ones, where disk I/O occurs.
 
 \begin{figure}
 \centering