[docs] : add Markdown format for '--time'

Change-Id: I9ddff9ae4234b6e83fc29f4a473b58211b00d546
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/5460
Reviewed-by: Scott Graham <scottmg@google.com>
Commit-Queue: Scott Graham <scottmg@google.com>

docs/quick_start.md

diff --git a/docs/quick_start.md b/docs/quick_start.md
index 629b36d..4e1bdaf 100644
--- a/docs/quick_start.md
+++ b/docs/quick_start.md
@@ -352,7 +352,7 @@
 
 ### Performance
 
-You can see what took a long time by running it with the --time command
+You can see what took a long time by running it with the `--time` command
 line flag. This will output a summary of timings for various things.
 
 You can also make a trace of how the build files were executed: