Fix URL on the go get instructions.

[mtpereira]

Hey!

I was trying to fetch the code by using your go get instructions and I noticed this small typo.

Thanks for the project!

[pbnj]

I banged my head against a problem for 1 hour because of this mis-spell.
The username mis-spell in README causes the following error:

github.com/erroneouboat/slack-term (download)
# cd .; git clone https://github.com/erroneouboat/slack-term /Users/pbenjamin/golang/src/github.com/erroneouboat/slack-term
Cloning into '/Users/pbenjamin/golang/src/github.com/erroneouboat/slack-term'...
fatal: could not read Username for 'https://github.com': terminal prompts disabled
package github.com/erroneouboat/slack-term: exit status 128