distcheck had only one possible invocation

That is, calling it an example is misleading

src/doc/rustc-dev-guide/src/tests/intro.md

@@ -116,7 +116,9 @@ This requires building all of the documentation, which might take a while.
 `distcheck` verifies that the source distribution tarball created by the build
 system will unpack, build, and run all tests.
 
-> Example: `./x test distcheck`
+```console
+./x test distcheck
+```
 
 ### Tool tests