fix typo (redis#1665)

Author: oranagra

topics/modules-api-ref.md

@@ -1348,7 +1348,7 @@ the string, and later call StringDMA() again to get the pointer.
 
     int RedisModule_StringTruncate(RedisModuleKey *key, size_t newlen);
 
-If the string is open for writing and is of string type, resize it, padding
+If the key is open for writing and is of string type, resize it, padding
 with zero bytes if the new length is greater than the old one.
 
 After this call, [`RedisModule_StringDMA()`](#RedisModule_StringDMA) must be called again to continue