Fix swarm manager node fault tolerance table

In a cluster of 4 nodes, with a majority being 3 nodes, the fault
tolerance should be 1 node.

Signed-off-by: Adam Walz <adam@adamwalz.net>

docs/swarm/admin_guide.md

@@ -64,7 +64,7 @@ guaranteed if you encounter more than two network partitions.
 |      1       |     1      |         0         |
 |      2       |     2      |         0         |
 |    **3**     |     2      |       **1**       |
-|      4       |     3      |         2         |
+|      4       |     3      |         1         |
 |    **5**     |     3      |       **2**       |
 |      6       |     4      |         2         |
 |    **7**     |     4      |       **3**       |