fix typo in readme for dockerfile building

README.md

@@ -117,7 +117,7 @@ the default one. Normal Docker behavior applies, so:
 If you're using a custom Dockerfile, you can use `target.{{TARGET}}.dockerfile` to automatically build it
 
 ``` toml
-[target.aarch64-unknown-linux-gnu.dockerfile]
+[target.aarch64-unknown-linux-gnu]
 dockerfile = "./path/to/where/the/Dockerfile/resides"
 ```