Update expect messages in library/alloc/src/io/buffered/bufreader.rs and library/alloc/src/io/copy/generic.rs to follow the style guide #159882

[rockysnow7]

Related issue: #159751

Updates the expect messages in library/alloc/src/io/buffered/bufreader.rs and library/alloc/src/io/copy/generic.rs to follow the style guide

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[clarfonthey]

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