Fix a typo in the spec

proposal-docs/alternateEnclosure/alternateEnclosure.md

@@ -31,7 +31,7 @@ Lastly, I propose optional information to verify integrity of downloaded media v
       rel="[(string)]"
       codecs="[(string)]"
       default="[(boolean)]">
-    [one or more of <podcst:source> and <podcast:integrity>]
+    [one or more of <podcast:source> and <podcast:integrity>]
     </podcast:alternateEnclosure>
     ```
 
@@ -184,4 +184,4 @@ Example use of the "rel" attribute offering "Behind the Scenes" content:
     <podcast:source uri="https://example.com/master.m3u8" />
     <podcast:source uri="ipfs://exampleLinkThatDoesntWorkHLS" />
 </podcast:alternateEnclosure>
-```
+```