[hotfix][docs] Add missed enumeration in MySQL Source

This closes #3832

Author: ChaomingZhangCN

flink-cdc-connect/flink-cdc-pipeline-connectors/flink-cdc-pipeline-connector-mysql/src/main/java/org/apache/flink/cdc/connectors/mysql/source/MySqlDataSourceOptions.java

@@ -126,8 +126,8 @@ public class MySqlDataSourceOptions {
                     .defaultValue("initial")
                     .withDescription(
                             "Optional startup mode for MySQL CDC consumer, valid enumerations are "
-                                    + "\"initial\", \"earliest-offset\", \"latest-offset\", \"timestamp\"\n"
-                                    + "or \"specific-offset\"");
+                                    + "\"initial\", \"earliest-offset\", \"latest-offset\", \"timestamp\", "
+                                    + "\"specific-offset\" or \"snapshot\".");
 
     public static final ConfigOption<String> SCAN_STARTUP_SPECIFIC_OFFSET_FILE =
             ConfigOptions.key("scan.startup.specific-offset.file")