docs: align constants/unicode with namespace conventions

Rename second test description from `'the exported object contains
key-value pairs'` to `'the exported object contains constants'` (80% of
siblings use the latter wording).

Author: claude

lib/node_modules/@stdlib/constants/unicode/test/test.js

@@ -33,7 +33,7 @@ tape( 'main export is an object', function test( t ) {
 	t.end();
 });
 
-tape( 'the exported object contains key-value pairs', function test( t ) {
+tape( 'the exported object contains constants', function test( t ) {
 	var keys = objectKeys( ns );
 	t.strictEqual( keys.length > 0, true, 'has keys' );
 	t.end();