Doxygen: Avoid ambiguity in @dir directives

This commit was automatically generated with:

find src -name '*.dox' |xargs perl -i -pe 's{\@dir ([^/])}{\@dir /$1};'

src/feature/api/feature_api.dox

@@ -1,4 +1,4 @@
 /**
-@dir feature/api
+@dir /feature/api
 @brief feature/api
 **/