gopls/doc/features: tweak markdown

Previously the <a> seems to cause underlining of what follows; see
https://github.com/golang/tools/blob/master/gopls/doc/features/diagnostics.md.
This fix is kind of a stab in the dark.

Change-Id: Ic552faae8d03b3d49c1a913ef7e3a145add5cfc4
Reviewed-on: https://go-review.googlesource.com/c/tools/+/651096
Auto-Submit: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: adonovan

gopls/doc/features/diagnostics.md

@@ -51,7 +51,7 @@ build`. Gopls doesn't actually run the compiler; that would be too
 
 There is an optional third source of diagnostics:
 
-<a id='toggleCompilerOptDetails'/>
+<a id='toggleCompilerOptDetails'></a>
 
 - **Compiler optimization details** are diagnostics that report
   details relevant to optimization decisions made by the Go