[FAB-9890] Fix typo, :doc:`configtxgen`
[FAB-9890] Fix typo, :doc:`configtxgen`

In build_network.rst, :doc:`configtxgen` should be
:doc:`commands/configtxgen` for the doc link to work.

Change-Id: Id1262f3d1f3eff8b9510db49b222c0127b337e6c
Signed-off-by: Robert P. J. Day <01aa5cd9a29907856ec6ddeebcb643ede4c45b8b@crashcourse.ca>

xixuejia/fabric,muralisrini/fabric,stemlending/fabric,manish-sethi/fabric-sidedb,xixuejia/fabric,binhn/fabric,binhn/fabric,manish-sethi/fabric,xixuejia/fabric,manish-sethi/fabric-sidedb,binhn/fabric,stemlending/fabric,jimthematrix/fabric,manish-sethi/fabric-sidedb,muralisrini/fabric,manish-sethi/fabric-sidedb,muralisrini/fabric,stemlending/fabric,manish-sethi/fabric-sidedb,hyperledger/fabric,manish-sethi/fabric-sidedb,jimthematrix/fabric,stemlending/fabric,manish-sethi/fabric-sidedb,hyperledger/fabric,manish-sethi/fabric