[clang-tools-extra] [clang-tidy][modernize-use-starts-ends-with] Fix minor mistake in error message (PR #116132)

[Julian Schmidt via cfe-commits]

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5chmidti wrote:

Please also add a test for the case you're doing this for, i.e., one without `[!=]= 0`, so that these don't regress in the future

https://github.com/llvm/llvm-project/pull/116132