CI: Caching earth_relief_15m_g used in doctests (#4662)

Author: seisman

pygmt/helpers/caching.py

@@ -46,6 +46,7 @@ def cache_data() -> None:
         "@earth_relief_05m_p",
         "@earth_relief_10m_g",
         "@earth_relief_10m_p",
+        "@earth_relief_15m_g",
         "@earth_relief_30m_g",
         "@earth_relief_30m_p",
         "@earth_synbath_01d_g",