api: fix NanoCPUs casing in swagger

While the field in the Go struct is named `NanoCPUs`, it has a JSON label to
use `NanoCpus`, which was added in the original pull request (not clear what
the reason was); 846baf1fd3

Some notes:

- Golang processes field names case-insensitive, so when *using* the API,
  both cases should work, but when inspecting a container, the field is
  returned as `NanoCpus`.
- This only affects Containers.Resources. The `Limits` and `Reservation`
  for SwarmKit services and SwarmKit "nodes" do not override the name
  for JSON, so have the canonical (`NanoCPUs`) casing.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

api/swagger.yaml

@@ -560,7 +560,7 @@ definitions:
         format: "int64"
         minimum: 0
         maximum: 100
-      NanoCPUs:
+      NanoCpus:
         description: "CPU quota in units of 10<sup>-9</sup> CPUs."
         type: "integer"
         format: "int64"
@@ -5466,7 +5466,7 @@ paths:
                 MemorySwap: 0
                 MemoryReservation: 0
                 KernelMemory: 0
-                NanoCPUs: 500000
+                NanoCpus: 500000
                 CpuPercent: 80
                 CpuShares: 512
                 CpuPeriod: 100000