docs: fix headline structure in readme

Signed-off-by: Jan Kowalleck <jan.kowalleck@gmail.com>

Author: jkowalleck

README.md

@@ -67,7 +67,7 @@ cyclonedx-py             # call script
 python3 -m cyclonedx_py  # call python module CLI
 ```
 
-## Basic usage
+### Basic usage
 
 ```shellSession
 $ cyclonedx-py --help