Add missing word

Author: greg0ire

contributing/code/stack_trace.rst

@@ -27,7 +27,7 @@ displayed by default if the exception is not caught. When using Symfony,
 such exceptions go through a custom exception handler, which enhances
 them in various ways before displaying them according to the current
 Server API (CLI or not).
-This means a better way to get a stack trace when you do need the
+This means a better way to get a stack trace when you do not need the
 program to continue is to throw an exception, as follows:
 ``throw new \Exception();``