README.md fix

[tkidd77]

[lucor]

Thanks for the PR, but the v2 is needed as per go module support. Without it go will download and install the version v1 on the develop branch.

Probably this sentence is misleading: To install a preview of the v2 version or help in testing: should we cache in something like: To install the development version or help in testing: ?

Btw to test the differences:

$ go get github.com/lucor/fyne-cross/v2/cmd/fyne-cross@develop
go: found github.com/lucor/fyne-cross/v2/cmd/fyne-cross in github.com/lucor/fyne-cross/v2 v2.0.1-0.20200610180511-291917e62cf7
$ fyne-cross version                                          
fyne-cross version v2.0.1-0.20200610180511-291917e62cf7

$ go get github.com/lucor/fyne-cross/cmd/fyne-cross@develop
go: found github.com/lucor/fyne-cross/cmd/fyne-cross in github.com/lucor/fyne-cross v1.5.1-0.20200418151505-2553e2b3bf20
$ fyne-cross -version                                          
fyne-cross version v1.5.1-0.20200418151505-2553e2b3bf20