Fix error in documentation

Indexation does not fail if no timestamp provided when there is a default value defined in mapping.

docs/reference/mapping/fields/timestamp-field.asciidoc

@@ -122,5 +122,5 @@ You can also set the default value to any date respecting <<mapping-timestamp-fi
 }
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-If you don't provide any timestamp value, indexation will fail.
+If you don't provide any timestamp value, _timestamp will be set to this default value.