Fixed documentation for perldoc format

plugins/systemd/systemd_units

@@ -21,9 +21,10 @@ about and/or exclude units you don't care about.
 (Note that for failed units, default warning and critical values are set to 0 and 10, respectively. No other
 states have default levels set.)
 
-Value calculations for each state are made using the following algorithm (all filters performed using \`egrep\`):
+Value calculations for each state are made using the following algorithm (all filters performed using
+C<grep -E>):
 
-1. Global include rules are applied on the output of \`systemctl list-units --all --no-legend\`;
+1. Global include rules are applied on the output of C<systemctl list-units --all --no-legend>;
 2. Global exclude rules are then applied to the result of that;
 3. Then, for each state, this global output is further filtered by include, then exclude rules for the state;
 4. Then the result is filtered for the given state and the remaining units counted and listed.