[fix compile error] Remove incorrect @return documentation from CreateStringImpl function

[zct]

compiler: clang++
version: 21.1.0

flatbuffers/include/flatbuffers/flatbuffer_builder.h:1400:8: error: '@return' command used in a comment that is attached to a function returning void [-Werror,-Wdocumentation]
1400 | /// @return Returns the offset in the buffer where the string starts.
| ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated

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