fixup! dev: Write the new include convention

Author: jorisv

development/convention.md

@@ -7,7 +7,7 @@ Pinocchio has 4 kinds of headers:
 - Private headers:
    - Definition: `xxx-def.hxx`
    - Declaration: `xxx-decl.hxx`
-   - Explicit template instantiation: `xxx-tpl.hxx`
+   - Explicit template instantiation: `xxx-expl.hxx`
 
 Since Pinocchio is mostly a header only library, include cycles are easy to create.