ebnflint: exit with non-zero status on error

Tweak spec to avoid ebnflint complaints.

R=gri
CC=golang-dev
https://golang.org/cl/3973050
diff --git a/doc/go_spec.html b/doc/go_spec.html
index 2d7f776..8707591 100644
--- a/doc/go_spec.html
+++ b/doc/go_spec.html
@@ -4078,11 +4078,11 @@
 CommClause = CommCase ":" { Statement ";" } .
 CommCase = "case" ( SendExpr | RecvExpr) | "default" .
 SendExpr =  Expression "<-" Expression .
+RecvExpr =  [ Expression ( "=" | ":=" ) ] "<-" Expression .
+</pre>
 <!-- TODO(rsc):
 RecvExpr =  [ Expression [ "," Expression ] ( "=" | ":=" ) ] "&lt;-" Expression .
 -->
-RecvExpr =  [ Expression ( "=" | ":=" ) ] "&lt;-" Expression .
-</pre>
 
 <p>
 For all the send and receive expressions in the "select"