Fix incorrect bootstrap invocation in README (#15)

The bootstrap used to be invoked using `MyFactory.init` but nowadays we changed it to accepting an factory instance.

**Problem:**

- snippet was misleading and would not compile

**Solution:**

- change to snippet that will work with current shape of API
- make the code snippet more visible and not in-line, so it is easier to spot "so what do I need to do to bootstrap again...?"

README.md

@@ -46,7 +46,13 @@ Note: If you are building a library, you don't need to concern yourself with thi
 
 SwiftMetrics only provides the metrics system API. As an application owner, you need to select a metrics backend (such as the ones mentioned above) to make the metrics information useful.
 
-Selecting a backend is done by adding a dependency on the desired backend client implementation and invoking `MetricsSystem.bootstrap(SelectedMetricsImplementation.init)` at the beginning of the program. This instructs the `MetricsSystem` to install `SelectedMetricsImplementation` (actual name will differ) as the metrics backend to use.
+Selecting a backend is done by adding a dependency on the desired backend client implementation and invoking the `MetricsSystem.bootstrap` function at the beginning of the program: 
+
+```swift 
+MetricsSystem.bootstrap(SelectedMetricsImplementation())
+```
+
+This instructs the `MetricsSystem` to install `SelectedMetricsImplementation` (actual name will differ) as the metrics backend to use.
 
 As the API has just launched, not many implementations exist yet. If you are interested in implementing one see the "Implementing a metrics backend" section below explaining how to do so. List of existing SwiftMetrics API compatible libraries: