gopls/doc/features: add "MCP" to index.

I sought and did not find.

Change-Id: I78d5b3286c9a494996a7d0852de399cbaec86d85
Reviewed-on: https://go-review.googlesource.com/c/tools/+/694117
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>

Author: adonovan

gopls/doc/features/index.md

@@ -64,7 +64,7 @@ when making significant changes to existing features or when adding new ones.
   - [go.mod and go.work files](modfiles.md): Go module and workspace manifests
   - [Go *.s assembly files](assembly.md): Go assembly files
 - [Command-line interface](../command-line.md): CLI for debugging and scripting (unstable)
-- [Model Context Protocol](mcp.md): use some features in AI-assisted environments
+- [Model Context Protocol (MCP)](mcp.md): use some features in AI-assisted environments
 
 You can find this page from within your editor by executing the
 `gopls.doc.features` [code action](transformation.md#code-actions),