docs(guide/Conceptual Overview): use exchangeratesapi.io

As fixer.io introduced an API key and thus a limitation of calls
(see https://github.com/fixerAPI/fixer#readme ),
change https://api.fixer.io to https://api.exchangeratesapi.io instead,
which is "designed and tested to handle thousands of request per second"
and has "built in Fixer.io compatibility so you can keep all the libraries you already like
and use daily" (from https://api.exchangeratesapi.io ).
The idea is from fixerAPI/fixer#107

fixes  #16807

Closes #16808

Author: bkiselka

docs/content/guide/concepts.ngdoc

@@ -186,7 +186,7 @@ Right now, the `InvoiceController` contains all logic of our example. When the a
 is a good practice to move view-independent logic from the controller into a
 <a name="service">{@link services service}</a>, so it can be reused by other parts
 of the application as well. Later on, we could also change that service to load the exchange rates
-from the web, e.g. by calling the [Fixer.io](http://fixer.io) exchange rate API, without changing the controller.
+from the web, e.g. by calling the [exchangeratesapi.io](https://exchangeratesapi.io) exchange rate API, without changing the controller.
 
 Let's refactor our example and move the currency conversion into a service in another file:
 
@@ -300,7 +300,7 @@ to something shorter like `a`.
 
 ## Accessing the backend
 
-Let's finish our example by fetching the exchange rates from the [Fixer.io](http://fixer.io) exchange rate API.
+Let's finish our example by fetching the exchange rates from the [exchangeratesapi.io](https://exchangeratesapi.io) exchange rate API.
 The following example shows how this is done with AngularJS:
 
 <example name="guide-concepts-3" ng-app-included="true">
@@ -331,7 +331,7 @@ The following example shows how this is done with AngularJS:
         };
 
         var refresh = function() {
-          var url = 'https://api.fixer.io/latest?base=USD&symbols=' + currencies.join(",");
+          var url = 'https://api.exchangeratesapi.io/latest?base=USD&symbols=' + currencies.join(",");
           return $http.get(url).then(function(response) {
             usdToForeignRates = response.data.rates;
             usdToForeignRates['USD'] = 1;