Label: Users Options: Login name > Username (joomla#12073)

The language label "Change Login Name" is not consistent with the front-end label "Username".

This PR changes the language label to "Change Username" and the decription to "Allow users to change their Username when editing their profile."

The front-end login screens use the label "Username"
Login Component:
![login-component](https://cloud.githubusercontent.com/assets/1217850/18627996/579510ca-7e5e-11e6-9161-6268e9ccb213.png)

Login Module:
![login-module](https://cloud.githubusercontent.com/assets/1217850/18627997/579595ae-7e5e-11e6-8920-edf34631a3c7.png)

### Testing Instructions
#### Before the PR
Users > Manage > [Options] > [User Options]
The last entry is "Change Login Name" 
+ hover description "Allow users to change their Login name when editing their profile."

![login-back-end-options](https://cloud.githubusercontent.com/assets/1217850/18627995/5791ac32-7e5e-11e6-8b18-94a34372c422.png)


#### After the PR
Users > Manage > [Options] > [User Options]
The last entry is "Change Username" 
+ hover description "Allow users to change their Username when editing their profile."

![login-back-end-options-after](https://cloud.githubusercontent.com/assets/1217850/18627998/5799ceb2-7e5e-11e6-9a5e-234dcf2db14e.png)

administrator/language/en-GB/en-GB.com_users.ini

@@ -32,8 +32,8 @@ COM_USERS_CONFIG_FIELD_ALLOWREGISTRATION_DESC="If set to Yes, new Users are allo
 COM_USERS_CONFIG_FIELD_ALLOWREGISTRATION_LABEL="Allow User Registration"
 COM_USERS_CONFIG_FIELD_CAPTCHA_DESC="Select the captcha plugin that will be used in the registration, password and username reminder forms. You may need to enter required information for your captcha plugin in the Plugin Manager.<br />If 'Use Default' is selected, make sure a captcha plugin is selected in Global Configuration."
 COM_USERS_CONFIG_FIELD_CAPTCHA_LABEL="Captcha"
-COM_USERS_CONFIG_FIELD_CHANGEUSERNAME_DESC="Allow users to change their Login name when editing their profile."
-COM_USERS_CONFIG_FIELD_CHANGEUSERNAME_LABEL="Change Login Name"
+COM_USERS_CONFIG_FIELD_CHANGEUSERNAME_DESC="Allow users to change their Username when editing their profile."
+COM_USERS_CONFIG_FIELD_CHANGEUSERNAME_LABEL="Change Username"
 COM_USERS_CONFIG_FIELD_FRONTEND_LANG_DESC="If 'Frontend User Parameters' is set to 'Show', users will be able to select their Frontend language preference when registering."
 COM_USERS_CONFIG_FIELD_FRONTEND_LANG_LABEL="Frontend Language"
 COM_USERS_CONFIG_FIELD_FRONTEND_RESET_COUNT_DESC="The maximum number of password resets allowed within the time period. Zero indicates no limit."