Fix function name in USAGE for H5Pencode2() (#4519)

src/H5P.c

@@ -788,7 +788,7 @@ H5Pget_size(hid_t id, const char *name, size_t *size /*out*/)
     The encoding of property values will be done according to the file format
     setting in fapl_id.
  USAGE
-    herr_t H5Pencode(plist_id, buf, nalloc, fapl_id)
+    herr_t H5Pencode2(plist_id, buf, nalloc, fapl_id)
         hid_t plist_id;         IN: Identifier to property list to encode
         void *buf:              OUT: buffer to gold the encoded plist
         size_t *nalloc;         IN/OUT: size of buffer needed to encode plist