doc: fix inline emphasis warning

Fix a warning spotted by linux-next build (htmldocs):

Documentation/filesystems/porting.rst:1186: WARNING: Inline emphasis start-string without end-string. [docutils]

Introduced by commit

  88d5baf6 ("Change inode_operations.mkdir to return struct dentry *")

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 88d5baf6 ("Change inode_operations.mkdir to return struct dentry *")
Signed-off-by: Christian Brauner <brauner@kernel.org>