Remove empty line in flags reference documentation

Author: vienthuong

resources/references/core-reference/dal-reference/flags-reference.md

@@ -27,4 +27,3 @@ nav:
 | Since | The "Since" flag defines since which Shopware version the field is available.                                                                                                                                                                                                                                                                   |
 | WriteProtected | By setting the "WriteProtected" flag, write access via API can be restricted. This flag is mostly used to protect indexed data from direct writing via API.                                                                                                                                                                                     |
 | Immutable | By setting the "Immutable" flag, it indicates that the field is write-once and then read-only                                                                                                                                                                                     |
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