bug fix: spec dir flag (#595)

The cli spec dir flag name should be `specdir`, not `specs`, that should match to:

```go
specDirFlag := cli.StringFlag{Name: "specdir", Usage: "Directory to store specs, defaults to ./specs"}
```

in fission/main.go

Currently it cause the specific specs dir not to work.

```
$ fission spec apply --specdir <my spec dir>
Everything up to date.
```

fission/spec.go

@@ -115,7 +115,7 @@ type (
 )
 
 func getSpecDir(c *cli.Context) string {
-	specDir := c.String("specs")
+	specDir := c.String("specdir")
 	if len(specDir) == 0 {
 		specDir = "specs"
 	}