fix typo

"and" should be "an"

spec/expressions.md

@@ -84,7 +84,7 @@ The following sections describe for each construct in the language exactly when
 
 ### Types of constituent expressions
 
-When an operation is statically bound, the type of a constituent expression (e.g. a receiver, and argument, an index or an operand) is always considered to be the compile-time type of that expression.
+When an operation is statically bound, the type of a constituent expression (e.g. a receiver, an argument, an index or an operand) is always considered to be the compile-time type of that expression.
 
 When an operation is dynamically bound, the type of a constituent expression is determined in different ways depending on the compile-time type of the constituent expression: