Add a --context option

Signed-off-by: Sylvain Rabot <sylvain@abstraction.fr>

docs/cmd/flux_create_source_git.md

@@ -71,6 +71,7 @@ flux create source git [name] [flags]
 ### Options inherited from parent commands
 
 ```
+      --context string      kubernetes context to use
       --export              export in YAML format to stdout
       --interval duration   source sync interval (default 1m0s)
       --kubeconfig string   path to the kubeconfig file (default "~/.kube/config")