Markdown: fix SELF_CLOSING_TAGS.contains call

CID 29319 (#1 of 1): EC: Comparing incompatible types for equality (FB.EC_UNRELATED_TYPES)
1. defect: Call to com.google.common.collect.ImmutableSet<java.lang.String>.equals(String).

This should be .contains() to test the argument is a member of the
set.  By mistake I wrote .equals(), which is always false as a set
is never equal to a string.

Change-Id: Ic1908c47a37e6ad930f182aac4ce0a29f86452b1
1 file changed