Update C3551

[Rageking8]

Error message seems outdated although the file below is already up-to-date:

cpp-docs/docs/error-messages/compiler-errors-2/compiler-errors-c3500-through-c3999.md

Line 69 in c192538

|[Compiler error C3551](compiler-error-c3551.md)|if a trailing return type is used then the leading return type shall be the single type-specifier 'auto' (not '*type*')|

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[Jak-MS]

@TylerMSFT

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[TylerMSFT]

Thank you!

[TylerMSFT]

#sign-off
The sentence starts lowercase because it's a verbatim of the error message from the compiler, which starts lowercase :-|