Fix missing word

Co-authored-by: Caleb Cartwright <calebcartwright@users.noreply.github.com>

Author: joshtriplett

src/doc/style-guide/src/expressions.md

@@ -330,7 +330,7 @@ than at other binary operators.
 
 If line-breaking a binary operator (including assignment operators) where the
 first operand spans multiple lines, use the base indentation of the *last*
-line of the first , and indent relative to that:
+line of the first operand, and indent relative to that:
 
 ```rust
 impl SomeType {