Fix Stable release date of 0.58.0 (facebook#19751)

Summary:
See facebook#19715 (comment) for the details.

hramos
Closes facebook#19751

Differential Revision: D8466070

Pulled By: hramos

fbshipit-source-id: 569e84119ac927e393889ad2c2f38d2b982aa6e8

Author: ikesyo

Releases.md

@@ -17,7 +17,7 @@ React Native follows a monthly release train. Every month, a new branch created
 |  ...    |       ...           |      ...         |
 | 0.56.0  | 1st of June         | 1st of July      |
 | 0.57.0  | 1st of July         | 1st of August    |
-| 0.58.0  | 1st of August       | 1st of October   |
+| 0.58.0  | 1st of August       | 1st of September |
 | ...     | ...                 | ...              |
 
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