Fix cost model example typo (graphprotocol#715)

fix typo

The query was invalid, this fixes it

website/pages/en/network/indexing.mdx

@@ -735,7 +735,7 @@ Example query costing using the above model:
 | ---------------------------------------------------------------------------- | ------- |
 | { pairs(skip: 5000) { id } }                             | 0.5 GRT |
 | { tokens { symbol } }                                    | 0.1 GRT |
-| { pairs(skip: 5000) { id { tokens } symbol } } | 0.6 GRT |
+| { pairs(skip: 5000) { id } tokens { symbol } } | 0.6 GRT |
 
 #### Applying the cost model