Fix Spec field length

Updated to trim the prefix "." to calculate the
length.

Author: nmvk

pkg/generate/code/compare.go

@@ -564,7 +564,8 @@ func CompareStruct(
 		var compareConfig *ackgenconfig.CompareFieldConfig
 		// memberFieldPath contains the field path along with the prefix cfg.PrefixConfig.SpecField + "." hence we
 		// would need to substring to exclude cfg.PrefixConfig.SpecField + "." to get correct field config.
-		fieldConfig := fieldConfigs[memberFieldPath[len(cfg.PrefixConfig.SpecField) + 1 :len(memberFieldPath)]]
+		specFieldLen := len(strings.TrimPrefix(cfg.PrefixConfig.SpecField, "."))
+		fieldConfig := fieldConfigs[memberFieldPath[specFieldLen + 1: len(memberFieldPath)]]
 		if fieldConfig != nil {
 			compareConfig = fieldConfig.Compare
 		}