fix the argument of the goimports example

After the last code correction, the name of the argument is different from the name used in the code.
Was "timeoutms" became "timeout_ms"

Change-Id: Id2c94b9a39ea162a9102e20e33b5ffcd019c5225
GitHub-Last-Rev: 4a2d3c8
GitHub-Pull-Request: #301
Reviewed-on: https://go-review.googlesource.com/c/tools/+/309229
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
Trust: Rebecca Stambler <rstambler@golang.org>
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

gopls/doc/vim.md

@@ -164,7 +164,7 @@ a helper function in Lua:
 lua <<EOF
   -- …
 
-  function goimports(timeoutms)
+  function goimports(timeout_ms)
     local context = { source = { organizeImports = true } }
     vim.validate { context = { context, "t", true } }