Update docs.json

Update .json doxy to point to 5.7

Author: Amanda Butler

docs.json

@@ -373,7 +373,7 @@
         "hidden": true,
         "type": "doxygen",
         "source": "https://github.com/ARMmbed/mbed-os",
-        "branch": "mbed-os-5.6",
+        "branch": "mbed-os-5.7",
         "options": {
             "PROJECT_NAME": "Mbed OS Reference",
             "ENABLE_PREPROCESSING": "YES",