Fixes typos (#849)

unstable/parser.go

@@ -132,12 +132,12 @@ func (p *Parser) NextExpression() bool {
 }
 
 // Expression returns a pointer to the node representing the last successfully
-// parsed expresion.
+// parsed expression.
 func (p *Parser) Expression() *Node {
 	return p.builder.NodeAt(p.ref)
 }
 
-// Error returns any error that has occured during parsing.
+// Error returns any error that has occurred during parsing.
 func (p *Parser) Error() error {
 	return p.err
 }