PhpDoc: Replace non-existing return type map with array

Summary: `map` is not a valid type. See https://docs.phpdoc.org/guide/references/phpdoc/types.html

Test Plan: Read docs; run static code analysis.

Reviewers: O1 Blessed Committers, mainframe98

Reviewed By: O1 Blessed Committers, mainframe98

Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D26306

src/applications/phid/utils.php

···
  * Group a list of phids by type.
  *
  * @param   array $phids Array of PHIDs.
- * @return  map of phid type => list of phids
+ * @return  array Mapping of phid type => list of phids
  */
 function phid_group_by_type($phids) {
   $result = array();