Fix format error in subresource_filter README.md

Top level header was missing a space causing the markdown to be parsed
incorrectly.

Bug: 854334
Change-Id: Ia721d8adb009877ba1d3bf536091387bedade23f
Reviewed-on: https://chromium-review.googlesource.com/1106671
Commit-Queue: John Delaney <johnidel@google.com>
Reviewed-by: Asanka Herath <asanka@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569729}

components/subresource_filter/README.md

-#Subresource Filter
+# Subresource Filter
 The subresource_filter component deals with code that tags and filters
 subresource requests based on: