[nrf fromlist] snippets: wifi-credentials: Fix the heading

Add `s` that was inadvertantly missed out. Creates unnecessary
confusion.

Fixes zephyrproject-rtos/zephyr#106506

Upstream PR #: 107184

Signed-off-by: Ravi Dondaputi <ravi.dondaputi@nordicsemi.no>

Author: rado17

snippets/wifi/wifi-credentials/README.rst

@@ -1,7 +1,7 @@
 .. _snippet-wifi-credentials:
 
-Wi-Fi Credentials Snippet (wifi-credential)
-###########################################
+Wi-Fi Credentials Snippet (wifi-credentials)
+############################################
 
 .. code-block:: console