Fixed broken link

Author: Jan-Olof Hendig

doc/Type/Proc/Async.pod6

@@ -235,7 +235,7 @@ L<X::Proc::Async::MustBeStarted> exception is thrown.
 
 Closes the standard input stream of the external program. Programs that read
 from STDIN often only terminate when their input stream is closed. So if
-waiting for the promise from C<#method start> hangs (for a program opened for
+waiting for the promise from L<#method start> hangs (for a program opened for
 writing), it might be a forgotten C<close-stdin>.
 
 The C<Proc::Async> object must be created for writing (with