[MXNET-782] Fix Custom Metric Creation in R tutorial (#12117)
[MXNET-782] Fix Custom Metric Creation in R tutorial (#12117)

* fix tutorial

* install instructions

* fix typo

ForkedReposBak/mxnet,DickJC123/mxnet,tlby/mxnet,eric-haibin-lin/mxnet,szha/mxnet,yajiedesign/mxnet,tlby/mxnet,reminisce/mxnet,apache/incubator-mxnet,tlby/mxnet,eric-haibin-lin/mxnet,larroy/mxnet,reminisce/mxnet,yajiedesign/mxnet,szha/mxnet,larroy/mxnet,ForkedReposBak/mxnet,dmlc/mxnet,ForkedReposBak/mxnet,mbaijal/incubator-mxnet,mbaijal/incubator-mxnet,leezu/mxnet,tlby/mxnet,DickJC123/mxnet,eric-haibin-lin/mxnet,larroy/mxnet,sxjscience/mxnet,dmlc/mxnet,ptrendx/mxnet,yajiedesign/mxnet,zhreshold/mxnet,ForkedReposBak/mxnet,szha/mxnet,eric-haibin-lin/mxnet,mbaijal/incubator-mxnet,reminisce/mxnet,apache/incubator-mxnet,ForkedReposBak/mxnet,ptrendx/mxnet,ptrendx/mxnet,mbaijal/incubator-mxnet,reminisce/mxnet,yajiedesign/mxnet,tlby/mxnet,szha/mxnet,ptrendx/mxnet,leezu/mxnet,szha/mxnet,leezu/mxnet,zhreshold/mxnet,sxjscience/mxnet,ptrendx/mxnet,reminisce/mxnet,leezu/mxnet,yajiedesign/mxnet,sxjscience/mxnet,mbaijal/incubator-mxnet,tlby/mxnet,mbaijal/incubator-mxnet,reminisce/mxnet,yajiedesign/mxnet,ptrendx/mxnet,larroy/mxnet,reminisce/mxnet,ptrendx/mxnet,dmlc/mxnet,leezu/mxnet,yajiedesign/mxnet,ForkedReposBak/mxnet,eric-haibin-lin/mxnet,ptrendx/mxnet,ForkedReposBak/mxnet,apache/incubator-mxnet,szha/mxnet,eric-haibin-lin/mxnet,DickJC123/mxnet,eric-haibin-lin/mxnet,larroy/mxnet,mbaijal/incubator-mxnet,szha/mxnet,yajiedesign/mxnet,larroy/mxnet,leezu/mxnet,sxjscience/mxnet,tlby/mxnet,sxjscience/mxnet,sxjscience/mxnet,zhreshold/mxnet,ForkedReposBak/mxnet,ptrendx/mxnet,szha/mxnet,dmlc/mxnet,DickJC123/mxnet,dmlc/mxnet,mbaijal/incubator-mxnet,leezu/mxnet,mbaijal/incubator-mxnet,larroy/mxnet,dmlc/mxnet,zhreshold/mxnet,larroy/mxnet,szha/mxnet,dmlc/mxnet,reminisce/mxnet,zhreshold/mxnet,larroy/mxnet,reminisce/mxnet,sxjscience/mxnet,dmlc/mxnet,sxjscience/mxnet,apache/incubator-mxnet,tlby/mxnet,dmlc/mxnet,leezu/mxnet,zhreshold/mxnet,reminisce/mxnet,sxjscience/mxnet,zhreshold/mxnet,tlby/mxnet,leezu/mxnet,eric-haibin-lin/mxnet,tlby/mxnet,ptrendx/mxnet,zhreshold/mxnet,yajiedesign/mxnet,zhreshold/mxnet,ForkedReposBak/mxnet,eric-haibin-lin/mxnet