added link to billing center to the simulator chapter

docs/services/simulator.md

@@ -6,7 +6,7 @@ In this step, users generate the field library by selecting isotropic or anisotr
 
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-In this step, the generation of the field library, as described in the chapter [**_Electromagnetic Modeling_**](/docs/background/electromagnetic_modeling.md), is computed. Given that DTI data was provided, the user can use the drop-down to select which type of simulation they would like to run first, isotropic or anisotropic. Afterwards the computation can be initialized by hitting the ```Setup``` button. A pop-up will appear with the information about the estimated cost of this action which can be accepted using the ```Confirm``` button. Since TIP v3.0 runs on AWS servers and creating the field library is a rather computationally intense step, this part is a premium feature which needs credits to be executed. For instructions on how to acquire credits, please refer to chapter \textit{Quick-Start-Guide/Platform overview/Billing Center}
+In this step, the generation of the field library, as described in the chapter [**_Electromagnetic Modeling_**](/docs/background/electromagnetic_modeling.md), is computed. Given that DTI data was provided, the user can use the drop-down to select which type of simulation they would like to run first, isotropic or anisotropic. Afterwards the computation can be initialized by hitting the ```Setup``` button. A pop-up will appear with the information about the estimated cost of this action which can be accepted using the ```Confirm``` button. Since TIP v3.0 runs on AWS servers and creating the field library is a rather computationally intense step, this part is a premium feature which needs credits to be executed. For instructions on how to acquire credits, please refer to chapter [**_Billing Center_**](/docs/platform_introduction/billing_center.md)
 
 Once all the preparations are finished, the logging console will show the information that the window can be closed. Since the simulations will run for approximately 2.5 to 3 hours, users can come back later once the results are available.