protocol/horizon/operations: fix AllowTrust comment to use SetTrustLi…

…neFlags #3998

What
This PR fixes the deprecation comment for AllowTrust so that users can be directed to the right operation.

Why
TrustlineFlagsUpdated is an effect, not an operation.

Known limitations
N/A

protocols/horizon/operations/main.go

@@ -207,7 +207,7 @@ type ChangeTrust struct {
 	TrustorMuxedID uint64 `json:"trustor_muxed_id,omitempty,string"`
 }
 
-// Deprecated: use TrustlineFlagsUpdated instead.
+// Deprecated: use SetTrustLineFlags instead.
 // AllowTrust is the json resource representing a single operation whose type is
 // AllowTrust.
 type AllowTrust struct {