[tools] Fix type in GC eval script

R=hpayer@chromium.org
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/1872633003

Cr-Commit-Position: refs/heads/master@{#35357}

tools/eval_gc_time.sh

@@ -62,7 +62,7 @@ do
       LOGFILE=$2
       shift
       ;;
-    -f|--percentiles)
+    -p|--percentiles)
       PERCENTILES="--percentiles=$2"
       shift
       ;;