docs: vm: page_frags.rst: Fix the reference of stale function

The __netdev_alloc_frag() has been dropped by commit 7ba7aeab ("net:
Don't disable interrupts in napi_alloc_frag()"). So replace it with
netdev_alloc_frag().
Signed-off-by: Kevin Hao <haokexin@gmail.com>
Link: https://lore.kernel.org/r/20200508140924.18220-1-haokexin@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/vm/page_frags.rst

@@ -26,7 +26,7 @@ to be disabled when executing the fragment allocation.
 
 The network stack uses two separate caches per CPU to handle fragment
 allocation.  The netdev_alloc_cache is used by callers making use of the
-__netdev_alloc_frag and __netdev_alloc_skb calls.  The napi_alloc_cache is
+netdev_alloc_frag and __netdev_alloc_skb calls.  The napi_alloc_cache is
 used by callers of the __napi_alloc_frag and __napi_alloc_skb calls.  The
 main difference between these two calls is the context in which they may be
 called.  The "netdev" prefixed functions are usable in any context as these