docs: change VPP_WITH_SAMPLE_PLUGIN to SAMPLE_PLUGIN in sample plugin documentation
docs: change VPP_WITH_SAMPLE_PLUGIN to SAMPLE_PLUGIN in sample plugin documentation

The sample_plugin_doc.md file refers to VPP_WITH_SAMPLE_PLUGIN make
parameter, while the VPP makefile uses SAMPLE_PLUGIN instead.

Type: docs
Signed-off-by: Haggai Eran <950eb09aa5363d787c1def6eb81db661edcaf1be@gmail.com>
Change-Id: Id11090fcdc78b77e246da48d0d890865011f1132

FDio/vpp,chrisy/vpp,FDio/vpp,vpp-dev/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,vpp-dev/vpp,vpp-dev/vpp,chrisy/vpp,FDio/vpp,vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp