minor fix. Use ':' instead of '=' for SPF mechanisms and modifiers.

src/storage/zone.cr

@@ -316,7 +316,7 @@ class DNSManager::Storage::Zone
 			end
 
 			def to_bind9(io : IO)
-				io << "#{t.to_s.downcase}=#{v}"
+				io << "#{t.to_s.downcase}:#{v}"
 			end
 		end
 
@@ -367,7 +367,7 @@ class DNSManager::Storage::Zone
 				end
 
 				io << t.to_s.downcase
-				io << "=#{v}" if v != ""
+				io << ":#{v}" if v != ""
 			end
 		end