Fix string view index in UpdateFileContents() #4176

Job	Run time
KytheXrefs	0s
ClangTidy	1h 27m 57s
x86_64 test	13m 42s
KytheVerification	0s
arm64 test	12m 26s
MacOsBuild	19m 10s
x86_64 test-nortti	13m 42s
MacOsBuildDevTools	7m 53s
x86_64 test-c++20	16m 53s
PrepareVSPlugin	2m 0s
x86_64 test-c++23	16m 39s
VerifyFormatting	1m 39s
x86_64 smoke-test	14m 28s
WindowsBuild	37m 47s
arm64 smoke-test	15m 7s
x86_64 asan	22m 20s
x86_64 asan-clang	17m 50s
x86_64 compile	12m 10s
arm64 compile	7m 17s
x86_64 compile-clang	7m 32s
arm64 compile-clang	7m 31s
x86_64 compile-static	7m 53s
arm64 compile-static	7m 26s
x86_64 compile-static-clang	7m 10s
arm64 compile-static-clang	7m 37s
x86_64 coverage	18m 45s
x86_64 clean	7m 15s
arm64 clean	7m 9s
Build and deploy GitHub pages	8s
📦 Release	0s
	6h 37m 26s

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