Fix ignoring changes under resources in the stability checking script (#10929)

It turns out that this uses different input for excludes than the wpt
tests-affected script. The config file input is turned into a regexp
that tries to match the entire path, so "resources" would just match a
file called "resources" and "resources/**" is needed to match the
entire path.
diff --git a/check_stability.ini b/check_stability.ini
index f5ec030..51a79e6 100644
--- a/check_stability.ini
+++ b/check_stability.ini
@@ -6,5 +6,5 @@
 # Exhaustively validating such changes is highly resource intensive
 # (particularly in terms of execution time), making it impractical in most
 # cases.
-ignore_changes: resources
+ignore_changes: resources/**
 results_url: https://pulls.web-platform-tests.org/api/stability