doc: actually fix redirects for strictflexarrays1 & strictflexarrays3

doc/redirects.json

···
     "index.html#footnote-stdenv-find-inputs-location.__back.0"
   ],
   "strictflexarrays1": [
-    "doc/stdenv/stdenv.chapter.md#strictflexarrays1"
+    "index.html#strictflexarrays1"
   ],
   "strictflexarrays3": [
-    "doc/stdenv/stdenv.chapter.md#strictflexarrays3"
+    "index.html#strictflexarrays3"
   ],
   "tester-shfmt": [
     "index.html#tester-shfmt"