Update to Glossary Definition: Double Spend

[VincentPiper]

Please see the following issue for context: #1248
TLDR: I believe the definition for "double-spend" should not contain the words "spend" or "spent"
and should be more specific and clear to differentiate from the pre-Bitcoin definition of double-spend.

I am not proficient in Github or programming in general, so hopefully this pull request is
appropriate and done correctly. If this is successful, I will also attempt to create a new glossary term
for "spend/spent" and others that may be helpful.

[wbnns]

Unless others object, this will be squashed and merged on Sunday, December 18th.

@VincentPiper Thank you for taking the time to articulate your perspective in #1248 and for learning how to submit a pull request to make the change. Also, for fixing the typo you found.