Docs: Fix grammar typo in CLI Reference docs (supabase#41948)

Author: mathealves

apps/docs/spec/cli_v1_commands.yaml

@@ -1640,7 +1640,7 @@ commands:
 
       Creates a new migration file locally.
 
-      A `supabase/migrations` directory will be created if it does not already exists in your current `workdir`. All schema migration files must be created in this directory following the pattern `<timestamp>_<name>.sql`.
+      A `supabase/migrations` directory will be created if it does not already exist in your current `workdir`. All schema migration files must be created in this directory following the pattern `<timestamp>_<name>.sql`.
 
       Outputs from other commands like `db diff` may be piped to `migration new <name>` via stdin.
     examples: