fix -Wshadowing in String.cpp
fix -Wshadowing in String.cpp

Summary: Didn't check this with -Wshadow when I first wrote it.

Reviewed By: ranjeeth

Differential Revision: D2926047

fb-gh-sync-id: 8391986dee2d6b5698491bd2995f039468ec684d
shipit-source-id: 8391986dee2d6b5698491bd2995f039468ec684d

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