doc: Fix warning in Documentation/security/IMA-templates.rst

This patch fixes the warning:

Documentation/security/IMA-templates.rst:81: WARNING: Inline
substitution_reference start-string without end-string.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

Author: robertosassu

Documentation/security/IMA-templates.rst

@@ -78,7 +78,7 @@ descriptors by adding their identifier to the format string
  - 'iuid': the inode UID;
  - 'igid': the inode GID;
  - 'imode': the inode mode;
- - 'xattrnames': a list of xattr names (separated by |), only if the xattr is
+ - 'xattrnames': a list of xattr names (separated by ``|``), only if the xattr is
     present;
  - 'xattrlengths': a list of xattr lengths (u32), only if the xattr is present;
  - 'xattrvalues': a list of xattr values;