minor #10748 Fix syntax error in decorating services example (danrot)

This PR was merged into the 3.4 branch.

Discussion
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Fix syntax error in decorating services example

There was a small error in the example for decorating services. This won't bother anybody since it is only an example of generated code, but it still looks weird 🙂

#SymfonyConHackday2018

Commits
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1fcf754 Fix syntax error in example

Author: javiereguiluz

service_container/service_decoration.rst

@@ -239,6 +239,6 @@ the ``decoration_priority`` option. Its value is an integer that defaults to
 
 The generated code will be the following::
 
-    $this->services[Foo:class] = new Baz(new Bar(new Foo()));
+    $this->services[Foo::class] = new Baz(new Bar(new Foo()));
 
 .. _decorator pattern: https://en.wikipedia.org/wiki/Decorator_pattern