Adapt to moved readthedocs account (#1195)

* Adapt to moved readthedocs account

.gitmodules

@@ -3,7 +3,7 @@
 	url = https://github.com/google/benchmark
 [submodule "sphinx_rtd_theme"]
 	path = ext/sphinx_rtd_theme
-	url = https://github.com/rtfd/sphinx_rtd_theme.git
+	url = https://github.com/readthedocs/sphinx_rtd_theme.git
 [submodule "python/pybind11"]
 	path = python/pybind11
 	url = https://github.com/pybind/pybind11.git