modify DECRBY command doc

[PansonPanson]

original:

Decrements the number stored at key by decrement. If the key does not exist, it is set to 0 before performing the operation. An error is returned if the key contains a value of the wrong type or contains a string that can not be represented as integer. This operation is limited to 64 bit signed integers.

modify:

The DECRBY command reduces the value stored at the specified key by the specified decrement. If the key does not exist, it is initialized with a value of 0 before performing the operation. If the key's value is not of the correct type or cannot be represented as an integer, an error is returned. This operation is limited to 64-bit signed integers.