Fix typings for Account Sequence Number (#203)

The actual constructor only takes a sequence number of type string.

types/index.d.ts

@@ -4,7 +4,7 @@
 export {};
 
 export class Account {
-  constructor(accountId: string, sequence: string | number);
+  constructor(accountId: string, sequence: string);
   accountId(): string;
   sequenceNumber(): string;
   incrementSequenceNumber(): void;