[groupbyattrsprocessor] Fix go.mod to match package name (#2662)

go.mod file had this:

module github.com/open-telemetry/opentelemetry-collector-contrib/processor/groupbyattrs

but the package name is 'groupbyattrsprocessor'

This mis-match resulted in errors like:

$ go get github.com/open-telemetry/opentelemetry-collector-contrib/processor/groupbyattrsprocessor
go get: github.com/open-telemetry/opentelemetry-collector-contrib/processor/groupbyattrsprocessor@none updating to
	github.com/open-telemetry/opentelemetry-collector-contrib/processor/[email protected]: parsing go.mod:
	module declares its path as: github.com/open-telemetry/opentelemetry-collector-contrib/processor/groupbyattrs
	        but was required as: github.com/open-telemetry/opentelemetry-collector-contrib/processor/groupbyattrsprocessor

preventing the module being used on opentelemetry-collector-builder.

Naming the module 'groupbyattrsprocessor' in go.mod fixes this error.

processor/groupbyattrsprocessor/go.mod

@@ -1,4 +1,4 @@
-module github.com/open-telemetry/opentelemetry-collector-contrib/processor/groupbyattrs
+module github.com/open-telemetry/opentelemetry-collector-contrib/processor/groupbyattrsprocessor
 
 go 1.14