Update importprivkey named args documentation

Fixes #11462. Updated documentation for importprivkey function to use the correct name for the first argument.
Also updates a call to importprivkey to use named args in functional test.

test/functional/importprunedfunds.py

@@ -78,7 +78,7 @@ class ImportPrunedFundsTest(BitcoinTestFramework):
         assert_equal(balance2, Decimal('0.05'))
 
         #Import with private key with no rescan
-        self.nodes[1].importprivkey(address3_privkey, "add3", False)
+        self.nodes[1].importprivkey(privkey=address3_privkey, label="add3", rescan=False)
         self.nodes[1].importprunedfunds(rawtxn3, proof3)
         balance3 = self.nodes[1].getbalance("add3", 0, False)
         assert_equal(balance3, Decimal('0.025'))