Fix psalm incompatibility with class-string<object> notation

[blackbirds24]

Problem

Psalm 6 crashes with an uncaught DocblockParseException when scanning
src/Metadata/Discriminator.php due to the class-string<object> type
in the docblock:

Uncaught Psalm\Exception\DocblockParseException: array<string, class-string<object>> 
is not a valid type (class-string param can only target to named or callable objects)

This crash occurs during autoload scanning, before analysis even starts,
making it impossible to use Psalm on projects that depend on AutoMapper.

Versions affected:

• AutoMapper: 10.0.3
• Psalm: 6.15.1

Fix

Replace class-string<object> with class-string in the Discriminator
constructor docblock.

Since every PHP class implicitly extends object, these two types are
strictly equivalent — no type safety is lost.

[joelwurtz]

Fine with me, since it works on phpstan also

[joelwurtz]

Thanks