children: use double quotes for arguments

You can't use single quotes in cmd.exe. See be7ef03d7fb6.

hgext/children.py

@@ -49,8 +49,8 b' def children(ui, repo, file_=None, **opt'
 
     Please use :hg:`log` instead::
 
-        hg children => hg log -r 'children()'
-        hg children -r REV => hg log -r 'children(REV)'
+        hg children => hg log -r "children()"
+        hg children -r REV => hg log -r "children(REV)"
 
     See :hg:`help log` and :hg:`help revsets.children`.