prompt_last_invisible --> @prompt_last_invisible
prompt_last_invisible --> @prompt_last_invisible

There were two places where the (undefined) local variable or method was
called mistakely in place of the instance variable.

Tested-by: Mark Somerville <26cc3217be640e8220112c25628da6e11c78db95@pccl.info> [F17, MRI 2.0.0-preview2]
Signed-off-by: Mark Somerville <f1b5a91d4d6ad523f2610114591c007e75d15084@scottishclimbs.com>

rapid7/rb-readline,abinoam/rb-readline,ConnorAtherton/rb-readline