Fix “please format Go code with ‘gofmt -s'” issue in building Swarmkit

When you do modifications in Swarmkit source code, e.g., doc.go, sometimes executing “make” command will show the following errors:

# make
 fmt
doc.go
 please format Go code with 'gofmt -s'

But executing “gofmt -s” command on doc.go seems not take effect:

# gofmt -s doc.go
// Package swarmkit implements a framework for task orchestration.
package swarmkit
# make
 fmt
doc.go
 please format Go code with 'gofmt -s'
make: *** [fmt] Error 1

The solution should be running “go fmt” command on this file, then the build process is OK:

# go fmt doc.go
doc.go
# make
 fmt
 bin/swarmd
......