Update STYLE_GUIDE.md

Co-Authored-By: Milan Curcic <[email protected]>

Author: zbeekman

STYLE_GUIDE.md

@@ -11,7 +11,7 @@
 
 ## File naming conventions
 
-- Source files should contain at most one `program` or `module`/`submodule`.
+- 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 added. E.g., `string_class.f90` and `tests/test_string_class.f90`