gopls/doc/features: add missing `

Change-Id: I099daf78c07335965c88af18240609b6775a0e5f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/639355
Reviewed-by: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Hongxiang Jiang <[email protected]>

Author: h9jiang

gopls/doc/features/passive.md

@@ -233,7 +233,7 @@ Gopls reports the following token types:
 
 Gopls also reports the following standard modifiers:
 
-- `"defaultLibrary": predeclared symbols
+- `"defaultLibrary"`: predeclared symbols
 - `"definition"`: the declaring identifier of a symbol
 - `"readonly"`: for constants