Doc: Use sensible argument in `\printto` command

The "Scene Graph - Vulkan Under QML" example includes snippets of code
from the example source file(s) using a combination of the `\skipto` and
`\printto` QDoc commands. The former skips content until the line that
contains the argument passed to the command
(here, "class VulkanSquircle"). The latter prints lines up to, but not
including, the line that contains the argument passed to the command
(here, "public"). I suspect the author's intention was for QDoc to
output lines from the start of the class declaration until the public
access specifier in the member specification of the class. However, the
first instance of `public` QDoc encounters is the base-specifier of the
derived class, which is on the same line as the argument to the
`\skipto` command. Hence, there's nothing for QDoc to output.

Modifying the argument to include the colon that indicates the keyword
is for member-declarations ensures QDoc generates output as expected.

Fixes: QTBUG-116304
Pick-to: 6.6 6.5
Change-Id: Ibfc17fbacd10f902fc2a5f57873fe0990571535e
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>
Reviewed-by: Safiyyah Moosa <safiyyah.moosa@qt.io>

examples/quick/scenegraph/vulkanunderqml/doc/src/vulkanunderqml.qdoc

@@ -37,7 +37,7 @@
 
     \quotefromfile scenegraph/vulkanunderqml/vulkansquircle.h
     \skipto class VulkanSquircle
-    \printto public
+    \printto public:
 
     We then go on to declare public and private items: