Fix corpus response parsing on "document"

Change-Id: Ib305c6430e997f5afdf07310844fab6c7f6443ac
diff --git a/parser/grammar_parser.go b/parser/grammar_parser.go
index 7c949f8..a224181 100644
--- a/parser/grammar_parser.go
+++ b/parser/grammar_parser.go
@@ -156,7 +156,7 @@
 			// Check if this parenthesis is inside brackets (part of an identifier)
 			insideBrackets := false
 			bracketDepth := 0
-			for j := 0; j < i; j++ {
+			for j := range i {
 				if runes[j] == '[' {
 					bracketDepth++
 				} else if runes[j] == ']' {