_content/ref/mod: document '-o' flag for go mod vendor

For golang/go#47327

Change-Id: I8f6a4da541be4d95aa8b420b0294a0c1e78b8a0b
Reviewed-on: https://go-review.googlesource.com/c/website/+/362814
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Than McIntosh <thanm@google.com>

_content/ref/mod.md

@@ -1955,7 +1955,7 @@ explicitly via the `-compat` flag.
 Usage:
 
 ```
-go mod vendor [-e] [-v]
+go mod vendor [-e] [-v] [-o]
 ```
 
 The `go mod vendor` command constructs a directory named `vendor` in the [main
@@ -1991,6 +1991,10 @@ despite errors encountered while loading packages.
 The `-v` flag causes `go mod vendor` to print the names of vendored modules
 and packages to standard error.
 
+The `-o` flag (added in Go 1.18) causes `go mod vendor` to output the vendor
+tree at the specified directory instead of `vendor`. The argument can be either
+an absolute path or a path relative to the module root.
+
 ### `go mod verify` {#go-mod-verify}
 
 Usage: