Restore missing MMFF94 class-2 (2-2-2) stretch-bend parameter

mmffstbn.par was missing the class-2 stretch-bend entry for atom
types (2,2,2) (sp2 carbon center with both bonds BT-flagged) that
is present in the original MMFF94 parameter set. Without it,
class-2 C(sp2)-C(sp2)-C(sp2) angles fall through to the MMFFDFSB
default (0.30/0.30) instead of the fitted values (0.219/0.250).

Verified against an independent transcription of the original file
(Tinker's mmff94.prm): it is the only missing entry among 283,
with no value differences in the rest. Energy impact on the
Halgren validation suite (JIYJAC): stretch-bend 0.2368 kcal/mol vs
0.1858 from the original BatchMin program; matches to 4 decimals once the entry is restored.

Inserted after the class-1 (2,2,2) entry to preserve the canonical
CXBA ordering.

Fixes #3003

data/mmffstbn.par

@@ -62,6 +62,7 @@
 0   1    2    5     0.215     0.128   C94
 2   1    2   37     0.246     0.260   C94
 1   2    2    2     0.250     0.219   C94
+2   2    2    2     0.219     0.250   C94
 2   2    2    3     0.155     0.112   C94
 0   2    2    5     0.207     0.157   C94
 1   2    2    5     0.267     0.159   C94