Break up long line

Author: rylev

src/compiler-debugging.md

@@ -56,7 +56,8 @@ stack backtrace:
   37: rustc_driver::run_compiler
 ```
 
-If you set `debug = true`, you will get line numbers for the stack trace. Then the backtrace will look like this:
+If you set `debug = true`, you will get line numbers for the stack trace. 
+Then the backtrace will look like this:
 
 ```text
 stack backtrace: