doc: fix typo in 'is_meta_character'
doc: fix typo in 'is_meta_character'

PR #873
rust-lang/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang/regex,rust-lang/regex,rust-lang/regex,rust-lang-nursery/regex