Update msan instructions to show release build

The current documentation shows a debug build.
When you attempt that an assert is triggered suggesting
a release build.  Update the documentation to reflect that.

TBR=kjellander@chromium.org
BUG=libyuv:731
TEST=gn gen out/Deb^C "--args=is_debug=true is_msan=true"

Change-Id: Ib78e3a7859aa391ac359fa38234aaef0554d0df4
Reviewed-on: https://chromium-review.googlesource.com/663882Reviewed-by: Cheng Wang <wangcheng@google.com>

docs/getting_started.md

@@ -277,10 +277,10 @@ Then run:
 
 ## Sanitizers
 
-    gn gen out/Debug "--args=is_debug=true is_asan=true"
-    ninja -v -C out/Debug
+    gn gen out/Release "--args=is_debug=false is_msan=true"
+    ninja -v -C out/Release
 
-    Sanitizers available: tsan, msan, asan, ubsan, lsan
+    Sanitizers available: asan, msan, tsan, ubsan, lsan, ubsan_vptr
 
 ### Running Dr Memory memcheck for Windows