Hello TheKatherine,
Timur has it right on how to change it.
Some other comments on error message text in general.
* Just because messages can be changed, does not mean that they should.
* Messages are often "generic" because it is never known how many pieces of code will call them, or under what conditions. You might understand perfectly how and why a message is called for one scenario. But will it always be called under the same conditions? By the same piece of code? In the same location? How about in future versions?
* Short specific changes - like your text change - are better than longer, more details messages.
* Consider pointing out to Knowledge Documents or similar sources to give extra detail.
So you might maintain a Knowledge Document with common error messages and how to fix them.
These are typically easier to maintain than altering the application code. For a start, you can delegate this out to anyone with knowledge document rights. (You could combine this with an update to the Error Message, so that the Error message points to your documentation source.)
Thanks, Kyle_R.