doc: clean up QEMU networking doc
doc: clean up QEMU networking doc

Noticed a typo in the first sentence, then some missing articles further
down, and some other clarity and grammar edits came along.

Signed-off-by: David B. Kinder <c56c8b29908a5b2af1b63001268d73a0b7f972ba@intel.com>

galak/zephyr,nashif/zephyr,ldts/zephyr,mbolivar/zephyr,Vudentz/zephyr,kraj/zephyr,explora26/zephyr,finikorg/zephyr,kraj/zephyr,galak/zephyr,punitvara/zephyr,zephyriot/zephyr,Vudentz/zephyr,mbolivar/zephyr,ldts/zephyr,nashif/zephyr,kraj/zephyr,nashif/zephyr,explora26/zephyr,finikorg/zephyr,punitvara/zephyr,ldts/zephyr,zephyriot/zephyr,finikorg/zephyr,Vudentz/zephyr,ldts/zephyr,explora26/zephyr,galak/zephyr,Vudentz/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,nashif/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,Vudentz/zephyr,zephyriot/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,finikorg/zephyr,ldts/zephyr,finikorg/zephyr,mbolivar/zephyr,zephyriot/zephyr,kraj/zephyr,galak/zephyr,mbolivar/zephyr,zephyriot/zephyr