Documentation edits in split.R Examples

[feinleib]

This is related to issue #500. I initially commented there, but I'm creating a new issue because PR #502 already fixed that issue.

I have two more fixes in the Examples section of split.R documentation:

1. The comment for str_split_1() is missing the second underscore.

# If you want to split a single string, use `str_split1`

2. The names of the functions are written differently in each comment. These should be made consistent with each other.

# If you want to split a single string, use `str_split1`
# Use fixed to return a character matrix
# str_split_i extracts only a single piece from a string

My understanding of the relevant section of the Tidyverse Style Guide is that the correct format for these comments is backticked, with trailing parentheses:

`str_split_1()`     `str_split_fixed()`     `str_split_i()`

[hadley]

Would you like to do a PR to fix this?

[hadley]

Fixed by #526