bazel overlay: fix regression to "list of string" bpdocs type.

A recent change to Blueprint [1] added the feature to infer the type of
list elements to more than just "list of strings", but this also changed
the text to "list of string" (without s). This CL fixes the bazel
overlay, which uses "list of strings".

Test: m bazel_overlay && bazel query //...  --package_path=out/soong/bazel_overlay

[1]: https://android.googlesource.com/platform/build/blueprint/+/764a77191737d68820082af40cf5ddc98dd264a3%5E%21/#F1

Change-Id: I7522d15b1ce751807e1881ce1e94f068d226b316