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 <vsemozhetbyt@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

.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