Update migration guide to new approach

The validator migration tool has been moved to its own repository, this
commit updates the migration guide to account for this. Oh, and it also
fixes a small typo :-).

build/migration-guide.md

@@ -19,12 +19,10 @@ The Validator Registration tool facilitates the pre-registration process for val
 - Keys and corresponding address generation
 - Validator registration via transactions
 
- First, you need to clone the [Nimiq CoreJS repository](https://github.com/nimiq/core-js?tab=readme-ov-file#quickstart) and follow the quickstart instructions.
+ First, you need to clone the [Nimiq Validator Registration Tool repository](https://github.com/nimiq/validator-registration-tool) and install its dependencies by executing `yarn` inside the repository.
 
 ### Step 1: Generate validator keys
 
-After you cloned the core-js repository, change directory to where the tool is located with ```cd clients/nodejs```.
-
 Execute the tool without parameters to generate a validator address, signing key, and voting key:
 
 ```shell
@@ -105,9 +103,9 @@ Ensure that you set your `sync_mode` as `full` or `history` in the consensus par
 
 The Activation tool establishes a connection with the PoW chain via RPC, extracting data from your configuration file and crosschecking it with the information in the JSON file within the PoW chain.
 
-To execute the tool successfully, you need to ensure that you are fully synced and in consensus **within the PoW chain**. This requires starting the PoW client with a RPC server enabled on the imported validator address(this might take a while).
+To execute the tool successfully, you need to ensure that you are fully synced and in consensus **within the PoW chain**. This requires starting the PoW client with a RPC server enabled on the imported validator address (this might take a while).
 
-You can start the RPC server by running the following command:
+You can start the RPC server by cloning the [Nimiq CoreJS repository](https://github.com/nimiq/core-js?tab=readme-ov-file#quickstart) then following the quickstart instructions and finally running the following command:
 
 ```shell
 node clients/nodejs/index.js --dumb --network=test --rpc=8648