By default, WordPress comes bundled with a filter hook called capital_P_dangit which is used to fix incorrect capitalization. Whenever you type the word “WordPress” with a lowercase p it automatically converts that p into an upppercase P.
However, if you prefer not to use this feature, you can remove it by adding a small amount of code to your functions.php file located inside your themes folder.
remove_filter( 'the_title', 'capital_P_dangit' ); remove_filter( 'the_content', 'capital_P_dangit' ); remove_filter( 'comment_text', 'capital_P_dangit' );
For more information : https://codex.wordpress.org/Function_Reference/capital_P_dangit