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

  88d5baf ("Change inode_operations.mkdir to return struct dentry *")

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 88d5baf ("Change inode_operations.mkdir to return struct dentry *")
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

Documentation/filesystems/porting.rst

@@ -1183,12 +1183,11 @@ LOOK_CREATE or LOOKUP_RENAME_TARGET.
 
 ** mandatory**
 
-->mkdir() now returns a 'struct dentry *'.  If the created inode is
-found to already be in cache and have a dentry (often IS_ROOT()), it will
-need to be spliced into the given name in place of the given dentry.
-That dentry now needs to be returned.  If the original dentry is used,
-NULL should be returned.  Any error should be returned with
-ERR_PTR().
+->mkdir() now returns a dentry.  If the created inode is found to
+already be in cache and have a dentry (often IS_ROOT()), it will need to
+be spliced into the given name in place of the given dentry. That dentry
+now needs to be returned.  If the original dentry is used, NULL should
+be returned.  Any error should be returned with ERR_PTR().
 
 In general, filesystems which use d_instantiate_new() to install the new
 inode can safely return NULL.  Filesystems which may not have an I_NEW inode