Doc: Rename snippet id to a unique identifier

The header file has two snippets with the same id. Rename to unique
values and link to the correct snippet id from the qdoc file.

Fixes: QTWEBSITE-1051
Pick-to: 6.3
Change-Id: I9003916f831898026f520fd86d60ab1c0230e6c4
Reviewed-by: Kai Koehne <kai.koehne@qt.io>

examples/qml/doc/src/qml-extending.qdoc

@@ -165,7 +165,7 @@ developed in the previous examples into two types - a \c Boy and a \c Girl.
 
 \section1 Declare Boy and Girl
 
-\snippet referenceexamples/coercion/person.h 0
+\snippet referenceexamples/coercion/person.h 1
 
 The Person class remains unaltered in this example and the Boy and Girl C++
 classes are trivial extensions of it. The types and their QML name are

examples/qml/referenceexamples/coercion/person.h

@@ -77,8 +77,7 @@ private:
     int m_shoeSize = 0;
 };
 
-
-// ![0]
+// ![1]
 class Boy : public Person
 {
     Q_OBJECT
@@ -97,6 +96,6 @@ public:
 };
 //! [girl class]
 
-// ![0]
+// ![1]
 
 #endif // PERSON_H