Improve the 'description' on the json.md page.

"Covers how to use the Protobuf to JSON conversion utilities" is not really what the page is.

PiperOrigin-RevId: 869198318

Author: protobuf-github-bot

content/programming-guides/json.md

@@ -1,7 +1,7 @@
 +++
 title = "ProtoJSON Format"
 weight = 62
-description = "Covers how to use the Protobuf to JSON conversion utilities."
+description = "Describes the spec details of the canonical JSON representation for Protobuf messages."
 type = "docs"
 +++