[DNR] Change listed availability of Modifyheaderinfo

The structs were already functional by M85, but because the
documentation was updated after the branch cut, the docserver listed
ModifyheaderInfo as being available at M86.

The availability is usually determined automatically by looking over API
schemas over different versions/channels in
chrome/common/extensions/docs/server2/availability_finder.py

Bug: 947591
Change-Id: Iae71ac3c7b6bc9fb7e7d86815e855e31f588f36e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2340074Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Commit-Queue: Kelvin Jiang <kelvinjiang@chromium.org>
Auto-Submit: Kelvin Jiang <kelvinjiang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795805}

chrome/common/extensions/docs/templates/json/api_availabilities.json

@@ -3,6 +3,10 @@
 // The versions listed here were drawn either from hardcoded HTML intro tables
 // or from the whats_new page.
 {
+  "declarativeNetRequest.ModifyHeaderInfo": {
+    "channel": "stable",
+    "version": 85
+  },
   "devtools.inspectedWindow": {
     "channel": "stable",
     "version": 18