cmd/go: fix typo in output of go help importpath

The output refers to 'go help modules-get' but the actual command is 'go
help module-get', singular.

Change-Id: Ie001f4181d80d3bf1995af2f257bf789dad5b33f
GitHub-Last-Rev: ce9b90e
GitHub-Pull-Request: #29605
Reviewed-on: https://go-review.googlesource.com/c/156737
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>

Author: mark-rushakoff

src/cmd/go/alldocs.go

@@ -266,7 +266,7 @@ listed in the GOPATH environment variable.
 (See 'go help gopath-get' and 'go help gopath'.)
 
 When using modules, downloaded packages are stored in the module cache.
-(See 'go help modules-get' and 'go help goproxy'.)
+(See 'go help module-get' and 'go help goproxy'.)
 
 When using modules, an additional variant of the go-import meta tag is
 recognized and is preferred over those listing version control systems.