Typo in README.md

[ssousa33-OS]

Just flagging a small typo that appears on both line 487 and 489 of README.md. mplementation -> implementation

proposal-structs/README.md

Lines 487 to 489 in 285ddde

	Strings in production engines have in-place mutation to transition representation in order to optimize for different use ases (e.g. ropes, slices, canonicalized, etc). Sharing strings will likely be the most challenging part of the mplementation.
	It is possible to support sharing strings by copying-on-sharing, but may be too slow. If possible, lockfree mplementations of in-place mutations above is ideal.