Markdown: fix <iframe frameborder="0"

Passing w to the rule meant the frameborder value was being
combined into the width attribute value, which may set the
width with an invalid value and cause the entire <iframe>
to be dropped.

Change-Id: Iace7665614f3901f6e7d3c13d32cc541e0ef93b8

gitiles-servlet/src/main/java/com/google/gitiles/doc/GitilesMarkdown.java

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/doc/GitilesMarkdown.java b/gitiles-servlet/src/main/java/com/google/gitiles/doc/GitilesMarkdown.java
index f077688..5494cd3 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/doc/GitilesMarkdown.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/doc/GitilesMarkdown.java
@@ -111,7 +111,7 @@
               sequence(string("src="), attribute(src)),
               sequence(string("height="), attribute(h)),
               sequence(string("width="), attribute(w)),
-              sequence(string("frameborder="), attribute(w))
+              sequence(string("frameborder="), attribute(b))
             ))),
         Spn1(), '>',
         Spn1(), string("</iframe>"),