Update metadata.md: classification field

[emmamarichal]

cc @m4rc1e CLASSIFICATION -> CLASSIFICATIONS According: https://github.com/googlefonts/gftools/blob/main/Lib/gftools/fonts_public.proto#L57

Written without s, we get:

☠ FATAL: Check METADATA.pb parse correctly. (com.google.fonts/check/metadata/parses)

* ☠ **FATAL** Family metadata at . failed to parse. TRACEBACK: 28:1 : Message type "google.fonts_public.FamilyProto" has no field named "classification". [code: parsing-error]

🔥 FAIL: Check for codepoints not covered by METADATA subsets. (com.google.fonts/check/metadata/unreachable_subsetting)

* 🔥 **FAIL** Could not parse metadata.pb file [code: unparsable-metadata]

[emmamarichal]

@m4rc1e could you review this? It's a very small correction