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>