fixes build error on mergeAllChunksOnShard (#3316)

Author: jmd-mongo

source/reference/command/mergeAllChunksOnShard.txt

@@ -40,7 +40,7 @@ The command takes the following fields:
 
 .. list-table::
    :header-rows: 1
-   :widths: 20 20 80
+   :widths: 20 20 30 30
 
    * - Field
      - Type