Fix signature of MIME in README

Relates to #293

Author: ileasile

docs/README-STUB.md

@@ -210,7 +210,7 @@ Other options are resolving library descriptor from a local file or from remote
 By default, the return values from REPL statements are displayed in the text form. To use richer representations, e.g.
  to display graphics or html, it is possible to send MIME-encoded result to the client using the `MIME` helper function: 
 ```kotlin
-fun MIME(vararg mimeToData: Pair<String, Any>): MimeTypedResult 
+fun MIME(vararg mimeToData: Pair<String, String>): MimeTypedResult 
 ```
 E.g.:
 ```kotlin

docs/README.md

@@ -244,7 +244,7 @@ Other options are resolving library descriptor from a local file or from remote
 By default, the return values from REPL statements are displayed in the text form. To use richer representations, e.g.
  to display graphics or html, it is possible to send MIME-encoded result to the client using the `MIME` helper function: 
 ```kotlin
-fun MIME(vararg mimeToData: Pair<String, Any>): MimeTypedResult 
+fun MIME(vararg mimeToData: Pair<String, String>): MimeTypedResult 
 ```
 E.g.:
 ```kotlin