Fix spelling.

Author: ehuss

src/expressions/block-expr.md

@@ -86,7 +86,7 @@ fn move_by_block_expression() {
 > _AsyncBlockExpression_ :\
 > &nbsp;&nbsp; `async` `move`<sup>?</sup> _BlockExpression_
 
-An *async block* is a variant of a block expression which evalutes to
+An *async block* is a variant of a block expression which evaluates to
 a *future*. The final expression of the block, if present, determines
 the result value of the future.