Fix out of scope variable in ghcmod#util#tocol

[jimenezrick]

Solves undefined variable error in line `return l:i + 1'.

Error detected while processing function <SNR>93_on_enter..338..ghcmod#util#tocol:
line    9:
E121: Undefined variable: l:i
Press ENTER or type command to continue
Error detected while processing function <SNR>93_on_enter..338..ghcmod#util#tocol:
line    9:
E15: Invalid expression: l:i + 1
Press ENTER or type command to continue
Error detected while processing function <SNR>93_on_enter..338..ghcmod#util#tocol:
line    9:
E121: Undefined variable: l:i

[eagletmt]

Thanks!