docs: fix license specifier and add hint to spdx (#664)

docs/recipe_file.md

@@ -881,7 +881,7 @@ the package server.
 ```yaml
 about:
   homepage: https://example.com/bsdiff4
-  license: BSD
+  license: BSD-3-Clause # (1)!
   license_file: LICENSE
   summary: binary diff and patch using the BSDIFF4-format
   description: |
@@ -890,6 +890,8 @@ about:
   documentation: https://docs.com
 ```
 
+1.  Only the SPDX specifiers are allowed, more info here: [SPDX](https://spdx.org/licenses/)
+
 ### License file
 
 Adds a file containing the software license to the package metadata.