Auto merge of #4119 - fabricedesre:patch-1, r=alexcrichton

Fix typo in argument name.

Author: bors

src/doc/external-tools.md

@@ -70,7 +70,7 @@ If you are using Rust, there is [cargo_metadata] crate.
 
 # Information about build
 
-When passing `--message=format=json`, Cargo will output the following
+When passing `--message-format=json`, Cargo will output the following
 information during the build:
 
 * compiler errors and warnings,