README.md: Fix typo on the word `versions`

Signed-off-by: Tiago Reis <reis.tiago@gmail.com>

README.md

@@ -20,7 +20,7 @@ go install tailscale.com/cmd/tailscale{,d}
 
 We only support the latest Go release and any Go beta or release
 candidate builds (currently Go 1.13.x or Go 1.14) in module mode. It
-might work in earlier Go vesions or in GOPATH mode, but we're making
+might work in earlier Go versions or in GOPATH mode, but we're making
 no effort to keep those working.
 
 ## Bugs