Docs: source profiles

use `source` for profiles since it's more explicit and harder to
miss than `.`

Author: ax3l

USAGE.rst

@@ -2,7 +2,7 @@
 
 .. seealso::
 
-   You need to have an :ref:`environment loaded <install-profile>` (``. $HOME/picongpu.profile``) that provides all :ref:`PIConGPU dependencies <install-dependencies>` to complete this chapter.
+   You need to have an :ref:`environment loaded <install-profile>` (``source $HOME/picongpu.profile``) that provides all :ref:`PIConGPU dependencies <install-dependencies>` to complete this chapter.
 
 Basics
 ======

docs/source/install/profile.rst

@@ -14,7 +14,7 @@ Ideally, store that file directly in your ``$HOME/`` and source it after connect
 
 .. code-block:: bash
 
-   . $HOME/picongpu.profile
+   source $HOME/picongpu.profile
 
 We listed some example ``picongpu.profile`` files below which can be used to set up PIConGPU's dependencies on various HPC systems.