docs: fix typo in firmware-related section

Fix a minor grammar issue by changing 'firmwares' to 'firmware' in the
Documentation/index.rst file.

Signed-off-by: Alexander Shatalin <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>
Message-ID: <[email protected]>

Author: sashok63

Documentation/index.rst

@@ -84,7 +84,7 @@ which are kept separately from the kernel's own documentation.
 Firmware-related documentation
 ==============================
 The following holds information on the kernel's expectations regarding the
-platform firmwares.
+platform firmware.
 
 .. toctree::
    :maxdepth: 1