Fix typo in CONTRIBUTING.md

modfications -> modifications

Signed-off-by: Hollow Man <[email protected]>

CONTRIBUTING.md

@@ -30,7 +30,7 @@ For details see below and https://sumo.dlr.de/docs/FAQ.html#how_do_code_contribu
 
 In order to allow the development of separate external modules for SUMO (which are not
 part of the mainline repository) every source code directory lists the files and 
-subdirectories where modfications need to be made available under the EPL and in a 
+subdirectories where modifications need to be made available under the EPL and in a 
 separate list which files / directories are considered interfaces to external modules,
 as an entry point see the [contribution README in the src dir](src/README_Contributing.md).