Change sub-expression to operand in a few places

Author: Havvy

src/expressions.md

@@ -48,10 +48,9 @@ value, and has effects during *evaluation*. Many expressions contain
 sub-expressions, called the *operands* of the expression. The meaning of each
 kind of expression dictates several things:
 
-* Whether or not to evaluate the sub-expressions when evaluating the expression
-* The order in which to evaluate the sub-expressions
-* How to combine the sub-expressions' values to obtain the value of the
-  expression
+* Whether or not to evaluate the operands when evaluating the expression
+* The order in which to evaluate the operands
+* How to combine the operands' values to obtain the value of the expression
 
 In this way, the structure of expressions dictates the structure of execution.
 Blocks are just another kind of expression, so blocks, statements, expressions,