flecsi: docs need py-sphinx-rtd-theme@:2 (spack#2578)

Author: rbberger

repos/spack_repo/builtin/packages/flecsi/package.py

@@ -96,7 +96,7 @@ class Flecsi(CMakePackage, CudaPackage, ROCmPackage):
 
     # FleCSI documentation dependencies
     depends_on("py-sphinx", when="+doc")
-    depends_on("py-sphinx-rtd-theme", when="+doc")
+    depends_on("py-sphinx-rtd-theme@:2", when="+doc")
     depends_on("py-recommonmark", when="@:2.2 +doc")
     depends_on("doxygen", when="+doc")
     depends_on("graphviz", when="+doc")