style: consistently use asterisks for bullets

STYLE_GUIDE.md

@@ -11,14 +11,15 @@
 
 ## File naming conventions
 
-- Source files should contain at most one `program`, `module`, or `submodule`.
-- The filename should match the program or module name and have the file extension `.f90` or `.F90` if preprocessing is required.
-- If the interface and implementation is split using submodules the implementation submodule file should have the same name as the
+* Source files should contain at most one `program`, `module`, or `submodule`.
+* The filename should match the program or module name and have the file extension `.f90` or `.F90` if preprocessing is required.
+* If the interface and implementation is split using submodules the implementation submodule file should have the same name as the
   interface (parent) module but end in `_implementation`.
   E.g., `string_class.f90` and `string_class_implementation.f90`
-- Tests should be added in the `tests` subdirectory and have the same name as the module they are testing with the `test_` prefix
+* Tests should be added in the `tests` subdirectory and have the same name as the module they are testing with the `test_` prefix
   added.
   E.g., `string_class.f90` and `tests/test_string_class.f90`
+* There should only ever be one `program` and `module` statement per file.
 
 ## Indentation & whitespace