Fix small grammar issue, this may change the meaning, please ensure this is the expected behavior (#1403)

Author: bogdandrutu

specification/context/api-propagators.md

@@ -94,7 +94,7 @@ Required arguments:
 
 Extracts the value from an incoming request. For example, from the headers of an HTTP request.
 
-If a value can not be parsed from the carrier for a cross-cutting concern,
+If a value can not be parsed from the carrier, for a cross-cutting concern,
 the implementation MUST NOT throw an exception and MUST NOT store a new value in the `Context`,
 in order to preserve any previously existing valid value.