chore(docs): Update button-checkbox.directive.ts (#1378)
chore(docs): Update button-checkbox.directive.ts (#1378)

Small API Ref description improvements
imribarr-compit/ng2-bootstrap,valor-software/ngx-bootstrap,sublime392/ng2-bootstrap,erodriguezh/ngx-bootstrap,valor-software/ng2-bootstrap,buchslava/angular2-bootstrap,felixkubli/ng2-bootstrap,Le0Michine/ng2-bootstrap,felixkubli/ng2-bootstrap,valor-software/ng2-bootstrap,IlyaSurmay/ngx-bootstrap-versioning,imribarr-compit/ng2-bootstrap,sublime392/ng2-bootstrap,felixkubli/ng2-bootstrap,IlyaSurmay/ngx-bootstrap-versioning,macroorganizm/ng2-bootstrap,imribarr-compit/ng2-bootstrap,sublime392/ng2-bootstrap,valor-software/ngx-bootstrap,buchslava/angular2-bootstrap,macroorganizm/ng2-bootstrap,Le0Michine/ng2-bootstrap,valor-software/ngx-bootstrap,valor-software/ngx-bootstrap,macroorganizm/ng2-bootstrap,buchslava/angular2-bootstrap