docs: update installing.rst (#2691)

`git submodule add` needs the branch before the repository or else it is ignored. The previous code checked out the `master` branch, not the `stable` branch.

Author: James Foster

docs/installing.rst

@@ -16,7 +16,7 @@ as a submodule. From your git repository, use:
 
 .. code-block:: bash
 
-    git submodule add ../../pybind/pybind11 extern/pybind11 -b stable
+    git submodule add -b stable ../../pybind/pybind11 extern/pybind11
     git submodule update --init
 
 This assumes you are placing your dependencies in ``extern/``, and that you are