[FIX] doc: Improve english

Some rephrasing to slightly improve the docs.
Closes #11652

doc/reference/guidelines.rst

@@ -16,7 +16,7 @@ Module structure
 
 Directories
 -----------
-A module is organised in a few directory :
+A module is organised in a few directories :
 
 - *data/* : demo and data xml
 - *models/* : models definition
@@ -266,7 +266,7 @@ Inside these 3 groups, the imported lines are alphabetically sorted.
 Idioms
 ------
 
-- Prefer ``%`` over ``.format()``, prefer ``%(varname)`` instead of position (This is better for translation)
+- Prefer ``%`` over ``.format()``, prefer ``%(varname)`` instead of position (This is better for translations)
 - Try to avoid generators and decorators
 - Always favor *Readability* over *conciseness* or using the language features or idioms.
 - Use list comprehension, dict comprehension, and basic manipulation using ``map``, ``filter``, ``sum``, ... They make the code easier to read.