Update README.md

Fixing a typo and slightly rewording, but this is based on the assumption that the rate limit is based on something like N requests per X minutes. I'm not 100% sure what it is based on and couldn't find documentation for it in my first pass of the docs.

README.md

@@ -95,7 +95,7 @@ All API calls return an err as their final return value. Below is documented the
 
 ### TooManyRequestsError
 
-`TooManyRequestsError` is returned when you exceed the manimum request that Mux allows. Please get in touch with [[email protected]](mailto:[email protected]) if you need to talk about this limit.
+`TooManyRequestsError` is returned when you exceed the maximum number of requests allowed for a given time period. Please get in touch with [[email protected]](mailto:[email protected]) if you need to talk about this limit.
 
 ### ServiceError