Fix a typo (#1286)

lang/spec.html

@@ -2775,7 +2775,7 @@ <h4>Singleton types</h4>
 </pre>
 <p>
 A singleton type is a type containing a single shape. A singleton type is
-described using an compile-time constant expression for a single value: the type
+described using a compile-time constant expression for a single value: the type
 contains the shape of that value. Note that it is possible for the
 variable-reference within the simple-const-expr to reference a structured value;
 in this case, the value will have its read-only bit set; a value without its