Add text language to code bloks in machine-readable-output.md

Travis fails when running `make doc` because of this file.
This should fix the issue.

src/doc/machine-readable-output.md

@@ -7,7 +7,7 @@ Cargo can output information about project and build in JSON format.
 You can use `cargo metadata` command to get information about project structure
 and dependencies. The output of the command looks like this:
 
-```
+```text
 {
  // Integer version number of the format.
  "version": integer,
@@ -59,7 +59,7 @@ without waiting for the whole build to finish.
 
 The message format looks like this:
 
-```
+```text
 {
  // Type of the message.
  "reason": "compiler-message",