doc: improve specificity in CODEOWNERS

Update CODEOWNERS entry for README.md to only apply to the top level
README.md. For example, if test/common/README.md is updated, that should
not be assigned to TSC automatically.

PR-URL: #20729
Reviewed-By: Vse Mozhet Byt <[email protected]>
Reviewed-By: Gus Caplan <[email protected]>
Reviewed-By: Jon Moss <[email protected]>
Reviewed-By: Yuta Hiroto <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>
Reviewed-By: Daniel Bevenius <[email protected]>
Reviewed-By: Michaël Zasso <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: James M Snell <[email protected]>

.github/CODEOWNERS

@@ -84,4 +84,4 @@ CONTRIBUTING.md @nodejs/tsc
 CPP_STYLE_GUIDE.md @nodejs/tsc
 GOVERNANCE.md @nodejs/tsc
 LICENSE @nodejs/tsc
-README.md @nodejs/tsc
+/README.md @nodejs/tsc