Use more explicit int-to-string conversion.
Use more explicit int-to-string conversion.

This fixes the following errors with Go 1.15:
```
src/options.go:452:69: conversion from untyped int to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
src/options.go:463:33: conversion from untyped int to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
```

junegunn/fzf,junegunn/fzf,junegunn/fzf,janlazo/fzf,janlazo/fzf,janlazo/fzf