pw_string: Fix reference to pw::InlineString
pw_string: Fix reference to pw::InlineString

The docs accidentally referred to it as pw::String.

Change-Id: I71437bb01f08a9ffc23ad844cf06804302df3aba
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/117833
Reviewed-by: Erik Gilling <1100d3bbb092e6691a138903fe8ea324306d03ea@google.com>
Pigweed-Auto-Submit: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>

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