Add -api-level to pm command

As of ~Aug 30th, `pm build` commands require an `api-level` flag. This flag should match the fuchsia api-level that's being targeted by the code. Since this is dependent on the version of the SDK that's being used, we may want to change this to something a bit more robust in the future.

Author: diminishedprime

src/doc/rustc/src/platform-support/fuchsia.md

@@ -414,6 +414,7 @@ Next, we'll build a package manifest as defined by our manifest:
 
 ```sh
 ${SDK_PATH}/tools/${ARCH}/pm \
+    -api-level 8 \
     -o pkg/hello_fuchsia_manifest \
     -m pkg/hello_fuchsia.manifest \
     build \