fix rules documentation

Documentation/Books/AQL/ExecutionAndPerformance/Optimizer.mdpp

@@ -136,7 +136,7 @@ Here is the meaning of these rules in context of this query:
 * `remove-unnecessary-calculations`: removes *CalculationNode*s whose result values are
   not used in the query. In the example this happens due to the `remove-redundant-calculations`
   rule having made some calculations unnecessary.
-* `use-index`: use an index to iterate over a collection instead of performing a
+* `use-indexes`: use an index to iterate over a collection instead of performing a
   full collection scan. In the example case this makes sense, as the index can be
   used for filtering and sorting.
 * `use-index-for-sort`: removes a `SORT` operation if it is already satisfied by
@@ -443,6 +443,7 @@ The following optimizer rules may appear in the `rules` attribute of a plan:
 * `inline-subqueries`: will appear when a subquery was pulled out in its surrounding scope,
   e.g. `FOR x IN (FOR y IN collection FILTER y.value >= 5 RETURN y.test) RETURN x.a`
   would become `FOR tmp IN collection FILTER tmp.value >= 5 LET x = tmp.test RETURN x.a`
+* `geo-index-optimizer`: will appear when a geo index is utilized (MMFiles-specific)
 
 The following optimizer rules may appear in the `rules` attribute of cluster plans: