Update docs.json

Switch order of targets page and thread safety page to match what readers want.

Author: Amanda Butler

docs.json

@@ -1002,11 +1002,11 @@
                 "title": "Porting targets",
                 "intro": {
                     "type": "markdown",
-                    "url": "https://github.com/ARMmbed/mbed-os-5-docs/blob/development/docs/reference/api/thread_safety.md"
+                    "url": "https://github.com/ARMmbed/mbed-os-5-docs/blob/development/docs/reference/contributing/target/target.md"
                 },
                 "sources": [{
                         "type": "markdown",
-                        "url": "https://github.com/ARMmbed/mbed-os-5-docs/blob/development/docs/reference/contributing/target/target.md"
+                        "url": "https://github.com/ARMmbed/mbed-os-5-docs/blob/development/docs/reference/api/thread_safety.md"
                     },
                     {
                         "type": "markdown",