Correct typo in development guide

Ref. None

doc/development_guide/index.rst

@@ -228,7 +228,7 @@ Code Design Guidelines
 The following guidelines should be followed for clarity and consistency throughout the project:
 
 - The formatting of error messages should be consistent. An error message starts with a lowercase letter and identifiers are highlighted by double quotes.
-- Private functions are prefered over inner functions. Long inner functions can impede the comprehension of a function.
+- Private functions are preferred over inner functions. Long inner functions can impede the comprehension of a function.
 - Internal methods are prefixed by a single underscore.
 - The methods of a class are sorted to make it easer to identify the public interface of a class:
    1. Special methods (starting and ending with double underscores)