[cw-ctl] fix help message for schemadiff In global help (closes #2888538)
The leading space make it appears on multiple line. The help content is a bit
improved in the process.
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html