docs: add simple invoke example

without this it isn't super clear that the name is a string and it's a member of the object list

docs/colls.rst

@@ -308,6 +308,11 @@ Data manipulation
 
     Calls named method with given arguments for each object in ``objects`` and returns an iterator or a list of results.
 
+    For example::
+
+        invoke(['abc', 'def', 'b'], 'find', 'b')
+        # ->[1, -1, 0]
+
 
 Content tests
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