build: clarify a .ignore rule

We weren't finding strings in tests/gold/html/Makefile

Author: nedbat

.ignore

@@ -1,7 +1,7 @@
 # .ignore for coverage: controls what files get searched.
 build/
 htmlcov/
-html0
+html0/
 .tox*
 .coverage*
 .metacov
@@ -10,7 +10,7 @@ coverage.xml
 coverage.lcov
 *.min.js
 style.css
-gold/
+tests/gold/*/*/*.*
 sample_html/
 sample_html_beta/
 *.so