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

Co-authored-by: Andrew Pollack <[email protected]>

Author: diminishedprime

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

@@ -414,7 +414,7 @@ Next, we'll build a package manifest as defined by our manifest:
 
 ```sh
 ${SDK_PATH}/tools/${ARCH}/pm \
-    -api-level 8 \
+    -api-level $(${SDK_PATH}/tools/${ARCH}/ffx version -v | grep "api-level" | head -1 |  awk -F ' ' '{print $2}') \
     -o pkg/hello_fuchsia_manifest \
     -m pkg/hello_fuchsia.manifest \
     build \