Dictionary: use the potentially clearer word "key" instead of "index" (#3724)

getting_started/scripting/gdscript/gdscript_basics.rst

@@ -804,7 +804,7 @@ for
 
 To iterate through a range, such as an array or table, a *for* loop is
 used. When iterating over an array, the current array element is stored in
-the loop variable. When iterating over a dictionary, the *index* is stored
+the loop variable. When iterating over a dictionary, the *key* is stored
 in the loop variable.
 
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