docs/usage: fix section 'trending source files'

The source code reference for the 'trending source files' dictionary was no
longer correct, so update it.
Additionally, some small rewording.

docs/usage/general.rst

@@ -113,10 +113,10 b' using @mention notation.'
 Trending source files
 ---------------------
 
-Trending source files are calculated based on a pre-defined dict of known
-types and extensions. If you miss some extension or would like to scan some
-custom files, it is possible to add new types in the ``LANGUAGES_EXTENSIONS_MAP`` dict
-located in ``kallithea/lib/celerylib/tasks.py``.
+Trending source files are calculated based on a predefined dictionary of known
+types and extensions. If an extension is missing or you would like to scan
+custom files, it is possible to extend the ``LANGUAGES_EXTENSIONS_MAP``
+dictionary located in ``kallithea/config/conf.py`` with new types.
 
 
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