`as_slice` -> `as_str` in the docs

[axil]

core::str::Str.as_slice() has been renamed to as_str() as discussed in #14433, but the old name is still referenced in the docs:

rust/src/libstd/primitive_docs.rs

Line 658 in 1863067

/// used to get a string slice under normal circumstances. Use `as_slice`