Updating description of a layer of operations.

[3yakuya]

Summary

Fixing layer description in "Implementing a BasicMapper Pass"

Details and comments

Original sentence stated "each layer is a group of operations that does not act on independent qubits, so in theory all operations in a layer can be done independently", which is incorrect. Every layer is a group of operations specifically acting on separate qubits each. The new sentence clarifies that with "each layer is a group of operations acting on independent qubits..."

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[MaldoAlberto]

Pardon that this took us so long to review! Thank you for making Qiskit better! Looks good to me.