debugging: slides: add space in flamegraph command for clarity

Signed-off-by: Alexis Lothoré <[email protected]>

Author: Tropicao

slides/debugging-system-wide-profiling/debugging-system-wide-profiling.tex

@@ -435,7 +435,7 @@ \subsection{perf}
     \item Format the data:
       \begin{block}{}
         \begin{minted}[fontsize=\small]{console}
-perf script|./fl/stackcollapse-perf.pl > out.perf-folded
+perf script | ./fl/stackcollapse-perf.pl > out.perf-folded
         \end{minted}
       \end{block}
       \begin{itemize}