Use no periods for partial phrases, like other providers

Note, this is fairly inconsistent across providers but when we see
inconsistences, we should try to pick which way to go and since most providers
have an incomplete sentence in these validation and helper texts, it makes
sense to try to keep this non-sentence structure.

I was noticing the english message catalog had the same partial sentence with
and without the period:

```
msgid "Must start with a letter and contain only letters, numbers, and underscores"
msgstr ""

msgid "Must start with a letter and contain only letters, numbers, and underscores."
msgstr ""
```

See: ManageIQ/manageiq#23665

Author: jrafanie

app/models/manageiq/providers/proxmox/infra_manager/vm/operations/snapshot.rb

@@ -14,7 +14,7 @@ def params_for_create_snapshot
           :id         => 'name',
           :label      => _('Name'),
           :isRequired => true,
-          :helperText => _('Must start with a letter and contain only letters, numbers, and underscores.'),
+          :helperText => _('Must start with a letter and contain only letters, numbers, and underscores'),
           :validate   => [
             {:type => 'required'},
             {:type => 'pattern', :pattern => '^[a-zA-Z][a-zA-Z0-9_]*$', :message => _('Must start with a letter and contain only letters, numbers, and underscores')}
@@ -34,7 +34,7 @@ def params_for_create_snapshot
           :onText     => _('Yes'),
           :offText    => _('No'),
           :isDisabled => current_state != 'on',
-          :helperText => _('Snapshotting the memory is only available if the VM is powered on.')
+          :helperText => _('Snapshotting the memory is only available if the VM is powered on')
         }
       ]
     }