Documentation: fix coding-style.rst Sphinx warning

Fix Sphinx warning in coding-style.rst:

Documentation/process/coding-style.rst:446: WARNING: Inline interpreted text or phrase reference start-string without end-string.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/process/coding-style.rst

@@ -443,7 +443,7 @@ In function prototypes, include parameter names with their data types.
 Although this is not required by the C language, it is preferred in Linux
 because it is a simple way to add valuable information for the reader.
 
-Do not use the `extern' keyword with function prototypes as this makes
+Do not use the ``extern`` keyword with function prototypes as this makes
 lines longer and isn't strictly necessary.