Fix code_coverage.md docs to specify is_debug=false GN arg.

One instance of this was already fixed in
https://chromium-review.googlesource.com/c/chromium/src/+/1874486.  This
CL simply covers the other.

Bug: NONE
Change-Id: Ic0dc4ce2fdc1ca0daa414b50d2f9cf5a59f676ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2130667
Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Sajjad Mirza <sajjadm@chromium.org>
Commit-Queue: Sajjad Mirza <sajjadm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755064}

docs/testing/code_coverage.md

@@ -130,7 +130,8 @@ Here is an example usage:
 
 ```
 $ gn gen out/coverage \
-    --args='use_clang_coverage=true is_component_build=false dcheck_always_on=true'
+    --args="use_clang_coverage=true is_component_build=false
+    dcheck_always_on=true is_debug=false"
 $ python tools/code_coverage/coverage.py \
     crypto_unittests url_unittests \
     -b out/coverage -o out/report \