server: Fix formatting of credential provider note

[dveeden]

Fixes this:

An alternative:

diff --git a/server/credential_provider.go b/server/credential_provider.go
index 3d44eb0..5333f89 100644
--- a/server/credential_provider.go
+++ b/server/credential_provider.go
@@ -4,7 +4,9 @@ import "sync"
 
 // interface for user credential provider
 // hint: can be extended for more functionality
+//
 // =================================IMPORTANT NOTE===============================
+//
 // if the password in a third-party credential provider could be updated at runtime, we have to invalidate the caching
 // for 'caching_sha2_password' by calling 'func (s *Server)InvalidateCache(string, string)'.
 type CredentialProvider interface {