Fix error message for duplicate end of input marker when merging subexpressions

[stanislav-tsaplev]

https://github.com/francisrstokes/super-expressive/blob/d35f5fae48e63d5fad21a859abec0535d8686d0c/index.js#L537 The function checks for absence of the end of the input, but the description mentions the start

[francisrstokes]

Nice spot. Since you spending some time going through the code, and noting these issues - please feel free to create pull requests to resolve them as well! I'd more than welcome the fixes.