update migration guide table in readme (#224)

MIGRATION.md

@@ -1,6 +1,6 @@
 # Migration
 
-## Migrating from v3 t v4
+## Migrating from v3 to v4
 
 The `graphite-threshold` check has now been removed in line with the deprecation of the Graphite service. We recommend removing this check from your code and following [Edge Delivery and Observability's guide to replacing Graphite](https://financialtimes.atlassian.net/wiki/spaces/DS/pages/8408989698/Migrating+an+app+away+from+Graphite-based+health+checks):
 

README.md

@@ -248,11 +248,12 @@ New versions of the module are published automatically by CI when a new tag is c
 
 ## Migration guide
 
-State | Major Version | Last Minor Release | Migration guide |
-:---: | :---: | :---: | :---:
-✨ active | 3 | N/A | [migrate to v3](MIGRATION.md#migrating-from-v2-to-v3) |
-╳ deprecated | 2.1 | N/A | [migrate to v2](MIGRATION.md#migrating-from-v1-to-v2) |
-╳ deprecated| 1 | 1.11 | N/A |
+State | Major Version | Last Minor Release |                    Migration guide                    |
+:---: |:-------------:| :---: |:-----------------------------------------------------:
+✨ active |       4       | N/A | [migrate to v4](MIGRATION.md#migrating-from-v3-to-v4) |
+╳ deprecated |       3       | N/A | [migrate to v3](MIGRATION.md#migrating-from-v2-to-v3) |
+╳ deprecated |      2.1      | N/A | [migrate to v2](MIGRATION.md#migrating-from-v1-to-v2) |
+╳ deprecated|       1       | 1.11 |                          N/A                          |
 
 
 ## Contact