Editorial: Replace "$x Contains $parseNode" with "$parseNode is contained within $x"

As seen at e.g. MightBothParticipate

Fixes #48

Author: gibson042

spec.html

@@ -271,7 +271,7 @@ <h1>
                 1. If _type_ is |UnaryExpression|, then
                   1. Set _unaryExpression_ to _candidate_.
                 1. Else if _type_ is |NumericLiteral|, then
-                  1. Assert: _unaryExpression_ Contains _candidate_ is *true*.
+                  1. Assert: _candidate_ is contained within _unaryExpression_.
                   1. Return _unaryExpression_.
                 1. Else,
                   1. Return _candidate_.