index.d.ts: Fix extends -> implements

Before:
```
node_modules/rn-fetch-blob/index.d.ts:147:43 - error TS2689: Cannot extend an interface 'EventTarget'. Did you mean 'implements'?

147 export declare class PolyfillBlob extends EventTarget {
                                              ~~~~~~~~~~~

error Command failed with exit code 1.
```

After:
```
✨  Done in 3.06s.
```

Author: jdanbrown

index.d.ts

@@ -144,7 +144,7 @@ export interface PolyfillProgressEvent extends EventTarget {
     total: number;
 }
 
-export declare class PolyfillBlob extends EventTarget {
+export declare class PolyfillBlob implements EventTarget {
     /**
      * RNFetchBlob Blob polyfill, create a Blob directly from file path, BASE64
      * encoded data, and string. The conversion is done implicitly according to