gopls/doc: fix imports function for Neovim LSP

The documented Lua function for supporting imports with Neovim native
LSP support does work for imports, but also runs any other code action
available in the current range. For example, if the function was to run
with the user's cursor on an empty struct being initialized, the code
action for filling that struct would also run.
This change restricts the desired actions to only be organizeImports.

Fixes golang/go#47181

Change-Id: Ie4eb5961cb4405091477b79175acc7052ff16d4c
Reviewed-on: https://go-review.googlesource.com/c/tools/+/334749
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Trust: Robert Findley <rfindley@google.com>

gopls/doc/vim.md

@@ -165,7 +165,7 @@ lua <<EOF
   -- …
 
   function goimports(timeout_ms)
-    local context = { source = { organizeImports = true } }
+    local context = { only = { "source.organizeImports" } }
     vim.validate { context = { context, "t", true } }
 
     local params = vim.lsp.util.make_range_params()