Fix documentation about force_update_password in rds_instance module (a…

…nsible-collections#957)

Fix documentation about force_update_password in rds_instance module

SUMMARY
Wrong name used in docs.
ISSUE TYPE

Docs Pull Request

COMPONENT NAME
plugins/modules/rds_instance.py

Reviewed-by: Joseph Torcasso <None>
Reviewed-by: Alina Buzachis <None>

plugins/modules/rds_instance.py

@@ -225,7 +225,7 @@
     master_user_password:
         description:
           - An 8-41 character password for the master database user. The password can contain any printable ASCII character
-            except "/", """, or "@". To modify the password use I(force_password_update). Use I(apply immediately) to change
+            except "/", """, or "@". To modify the password use I(force_update_password). Use I(apply immediately) to change
             the password immediately, otherwise it is updated during the next maintenance window.
         aliases:
           - password