README.md: fix example code & code markup

Summary:

• scope examples with ` and code type
• serviceByDesktopName is the correct method
• drop 0 parameter from createInstance call (nullptr is default arg)
• drop NO_MODULE from find_package(KF5TextEditor), not really needed, more simple example

Reviewers: Kate, cullmann

Reviewed By: Kate, cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D27512