Try to fix Sphinx warning in formulas.rst

Author: felicitymay

docs/codeql/ql-language-reference/formulas.rst

@@ -305,7 +305,7 @@ languages. Here is a brief overview:
 .. _true:
 
 ``any()``
-======
+=======
 
 The built-in predicate ``any()`` is a formula that always holds.