README.md: Correct link to Quick Start guide

gn's README.md referenced docs/getting_started.md; the introductory
guide to GN is in docs/quick_start.md

Change-Id: I5b1aa544218af42214a63926911730fc1bddc7e6
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/7920
Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>

README.md

diff --git a/README.md b/README.md
index 5b18370..805760c 100644
--- a/README.md
+++ b/README.md
@@ -6,8 +6,8 @@
 Related resources:
 
   * Documentation in [docs/](https://gn.googlesource.com/gn/+/master/docs/). In
-    particular [getting
-    started](https://gn.googlesource.com/gn/+/master/docs/getting_started.md)
+    particular [GN Quick Start
+    guide](https://gn.googlesource.com/gn/+/master/docs/quick_start.md)
     and the [reference](https://gn.googlesource.com/gn/+/master/docs/reference.md)
     (the latter is all builtin help converted to a single file).
   * An introductory [presentation](https://docs.google.com/presentation/d/15Zwb53JcncHfEwHpnG_PoIbbzQ3GQi_cpujYwbpcbZo/edit?usp=sharing).