fix: standardize header formatting in README

Author: junioreif

README.rst

@@ -87,7 +87,7 @@ The following libraries are **required** for installing **OpenSMOG**:
     - `lxml <https://lxml.de/>`__ (>=4.6.2)
 
 Installing/Configuring SMOG2
-================
+============================
 
 The inputs required for **OpenSMOG** simulations can be generated using `SMOG 2 <https://smog-server.org/smog2>`_ (version 2.4 and later). For a description of the various ways in which you may access SMOG 2 (e.g. Docker/Singularity container, conda, etc), see the README file in the `SMOG 2 GitHub repo <https://github.com/smog-server/SMOG2/>`__.