Update API in raftable README.

Author: kvap

contrib/raftable/README

@@ -36,26 +36,26 @@ The backend can also issue commands to itself through C API.
 
 C API:
 	/* Gets value by key. Returns the value or NULL if not found. */
-	char *raftable_get(int key);
+	char *raftable_get(char *key);
 
 	/*
 	 * Adds/updates value by key. Returns when the value gets replicated on
 	 * current machine. Storing NULL will delete the item from the table.
 	 */
-	void raftable_set(int key, char *value);
+	void raftable_set(char *key, char *value);
 
 	/*
 	 * Iterates over all items in the table, calling func(key, value, arg)
 	 * for each of them.
 	 */
-	void raftable_every(void (*func)(int, char *, void *), void *arg);
+	void raftable_every(void (*func)(char *, char *, void *), void *arg);
 
 SQL API:
 	-- set
-	raftable(key int, value text);
+	raftable(key varchar(64), value text);
 
 	-- get
-	raftable(key int) returns text;
+	raftable(key varchar(64)) returns text;
 
 	-- list
-	raftable() returns table (key int, value text);
+	raftable() returns table (key varchar(64), value text);