docs/style_guide: revert stance on non-type template args
docs/style_guide: revert stance on non-type template args

Reverts our stance on non-type template argument style to be
consistent with the naming standard applied to other varibles whose
value is fixed for the duration of the program, namely kCamelCase.

Change-Id: If3309ca63199cebaf24d058b82296000abf05a45
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/39660
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Ewout van Bekkum <638f134dbc68778d6b859fb04a64baea2b00b021@google.com>
Reviewed-by: Keir Mierle <69344103cc60b36d6e1fbdc31ce6504b2286c646@google.com>
Reviewed-by: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed